Garlic is involved in everything savoury I cook. 
I’m a bit late out of gate, but I think it’s a wonderful idea. During this thread you may have noticed there is considerable differences in how various Americans celebrate the holiday. For instance, the canned-cranberry-vs.-cranberry-relish divide, which I’m surprised doesn’t lead to an annual breakout of hostilities across the land. There are also regional differences, traditions specific to one’s family, and so forth.
As you are in the UK creating certain aspects of the America holiday will be nigh impossible. In the US the entire turkey-producing industry is geared to supplying massive, massive amounts of birds for the holiday - roughly 1 for every household, over 100 million birds. This affects both availability and price. This sort of thing just will not hapen in the UK. Although the cook getting stressed out over the dinner preperations is also very traditional I don’t recommend you adopt that particular tradition! As noted, Americans get the entire day off. It’s not uncommon to have a half day of work the day before. Many take extra days off, even the entire week. This just won’t happen for you.
So, my understanding is that you want something Thanksgiving that is actually doable in the time you and your guests have. And that is entirely possible.
I will say, though, that having massive, massive amounts of food is very traditional. This is, after all, based on the old harvest feasts. You’re supposed to eat yourself into a stupor. And having massive amounts of leftovers is also very traditional, as is insisting guests take some of the leftovers home. Really, it is. You can’t go to Thanksgiving dinner without the whole “Oh, PLEASE take some of this home with you!” routine. Unless this would be a gross breach of UK custom urge your guests to take at least a token amount home with them.
Also keep in mind that it is perfectly acceptable to be flexible on dietary requirements. I would also think that true American Thanksgiving menu might be a bit foreign to those in the UK, with foods that may not be commonly consumed, which might make people inclined to take smaller portions than they would over here. That, and for them it will be a dinner in a fairly normal day, unlike the US where it can become a multi-hour sole meal of the day.
I agree with folks who say that’s not enough turkey for a traditional Thanksgiving, and traditional is the whole bird. However, as I said, this may not be practical for you. The important thing is that there is turkey available for everyone - aside from those who simply don’t eat meat, the tradition is to take at least a small portion of turkey, even if you’re not terribly fond of it. It’s also traditional to have both white and dark meat. Under the circumstances, though, if white meat is a large cost savings by all means go with it, or if it fits your oven better than a whole bird would.
I agree that mashed potatoes are FAR more traditional that roasted. Mashed potatoes with gravy, to be specific. So much so I’d say “cheat” and use instant (yes, I hear howling from the purists, but we want you to cook dinner, not kill yourself via exhaustion). Truth is, cooks cheat over here, too - you can purchase entire roast birds which you then reheat (I’m assuming that won’t be an option over there), side dishes are often purchased already prepared at the grocery store, as are pies, and so on. While the ideal is to do everything at home from scratch there’s this thing called “reality” that often throws a [del]wrench[/del] spanner in the works.
The stuffing amount is on the low side for Thanksgiving, but if stuffing isn’t a common UK food you might want to opt for that quantity, as not everyone likes stuffing. Depending on how it’s made, it might also be an issue for the gluten-sensitive or vegetarians as well.
The onions and squash are a nice touch. The spinach and kale entirely appropriate.
The sweet potatoes with marshmallows IS very traditional - in SOME households. In other households they are considered an abomination. It’s another Thanksgiving Great Divide. Apparently you married into a sweet potato with marshmallows family, so I’m afraid you are [del]doomed[/del] stuck with this one. 
The pies are very traditional, as is the ice cream, but… sorbet? Not traditional. Not that there is anything wrong with that, it’s just not traditional.
The amount of bread seems low, but not all households are heavy bread eaters and, again, you and your guests are not used to the traditional quantities. As long as there is enough for everyone to get some you’re OK.
The Dreaded Cranberry Jelly…! I come from a canned cranberry family myself, but as noted it’s another source of conflict. If it is traditional in your family (that is, your husband’s family) or you can’t get or make the other kind go with it. The point is to have something cranberry to serve with the turkey along with the gravy. Oh, the gravy - yes, traditionally it’s made at home but this is another area where if you cheat I’ll forgive you.
You really do need another main entree (especially for the vegetarians!). Go with the macaroni and cheese with gluten-free noodles. Very American. It sounds like everyone can eat it.
Other items that fit are the previously mentioned green bean cassarole. Succotash, another American dish (it actually goes back to the Natives, with the name being derived from an indigenous language and not English) with many regional and family variations. Cornbread (not sure how common that is in the UK). Corn on the cob, which is easily boiled and served with butter and salt, but I’m not sure if that’s a common way of serving corn in the UK so obtaining it on the cob might not be as easy as here. Baked beans would not be out of place, either.
And - not that anyone is expecting you to do this! - in my family it was also traditional to have “relish” or snack trays set out for people to snack on as they arrived throughout the day. We usually had one that was various sausage and cheeses; at least one, and sometimes two or more, trays of raw vegetables; one of various crackers and breads; and a melon salad. So people would start showing up in the morning, socialize, help the cook, and snack through mid-day, then sit down to the feast and eat ourselves completely silly.
I think that I should mention that Thanksgiving is the traditional day to break news to the entire family. For instance, if someone’s finally coming out of the closet, Thanksgiving’s the day to announce it.
This ensures lively conversation around the table, and also ensures that half of the family won’t talk to the other half until NEXT Turkey Day.
Yes, and one of your guests needs to have a little too much to drink and start asking overly personal questions like, “When are you two finally going to have childeren?” And if you already have one it’s, “When are you going to have another one? LittleSecondJudith, could certainly use a brother/sister. I’ll bet his reading level would start finally going up if he/she had a sibling.”
It came out around actress Lynn Redgrave’s Thanksgiving table that her husband John Clark was the biological father of her personal assistant’s son. The personal assistant that Lynn & John’s son Benjamin later married and he adopted the boy (his half-brother).
I would have paid good money to be at that T-day feast.
If you have to ask if you have enough food for Thanksgiving… you don’t have enough food for Thanksgiving!
We’re going to my brother-in-law’s, and I’m bringing chocolate pecan pie and pumpkin praline pie (with gingersnap crust).
I wanted to make a pumpkin-gingerbread trifle, but that got nixed because 1. there are two other pumpkin desserts and 2. the host asked for pie. Maybe I’ll make the trifle for my co-workers.
Of all the things that finally stopped me procrastinating and got me posting rather than lurking, it had to be a debate on the size of a turkey.
I’m over in the UK too, and a couple of weeks ago, I decided to practice Xmas dinner, since I hadn’t done it in years, and it’s my turn this time round. I bought the Waitrose free range turkey breast crown (looks like the one you’re getting) and invited my sister and a friend, thinking there would be enough meat for the three of us, and a little bit of leftovers.
By Day Four of Eating Leftovers, the cats helped me throw the last of the meat in the bin, as even they had had enough turkey. I couldn’t believe how much meat was on the bird: there’s very little carcass in that 2.75kg weight since it’s just breast meat - I’d guess mine was about 500g carcass, and the rest was just meat. If my experience is anything to go by, you’ll have more than enough meat.
It also cooks really quickly as well, compared to a whole turkey, which suprised me somewhat.
Looking at the rest of your meal, the quantity is about what I’d serve for a group that size. If you’re worried about not having enough food, just serve them a cold starter, something you can prepare earlier.
And I really recommend the vaguely neurotic minute-by-minute planning. Iit’s the only thing that keeps me sane when I cook.
Of course the biggest problem with a turkey crown, is that it is most of the dark meat has been removed. The dark meat is really the only meat worth living for.
Yuck. I admit that the flavor of dark meat is so much better than the white meat. BUT, and this is a bit BUT, the texture is nasty.
Get a Kosher Ham. It’s very traditional for Thanksgiving.
Or cut up the potatoes, put them in a bowl of water, and nuke for 10 minutes. Drain the water and they should be ready to mash.
i’m genuinely shocked at the cognitive dissonance between the US and UK regarding thanksgiving. i guess we ugly americans really take this holiday for granted.
Also, i agree with Wendell Wagner in that the thanksgiving menu doesn’t lend itself to be tossed together after work. i’m pretty sure i’d laugh right at my gf’s face if she told me that i had to make enough figgy pudding, kidney pie, and beef wellington for 12 people on a random thursday after work.
why not try this on a saturday? it’ll be a more traditional, less hectic, and ultimately more enjoyable time to be had by all.
my suggestion to you is… stick with the turkey breast, make mac/cheese, make the mashed potatoes rustic (skin still on), and make stuffing from the box. the problem with stuffing is the gluten…
good luck.
Or move it to a saturday. If you really want the true thanksgiving experience you need to move it to a saturday, get the whole small turkey, and make the whole traditional menu with judicious additions for dietary needy people.
You might want to see if you can watch a food network thanksgiving meal program, let me check youtube fast.
Your dinner sounds delightful!
I was shocked to hear the price of a bird in the UK, till I remembered that they aren’t native to there. hehehe
How about going to a nice restaurant instead? 
Mashed potatoes: no problem. Peel large potatoes, cut up a bit, cook in boiling water for 20-30 mins (depending on how large you cut the chunks) mash up with butter, salt, pepper, cream or milk and (optional) some grated parmesan cheese. Done! Just cover and keep warm. Easy-peasy.
Yeah, they have to fly over, and their wings get tired.
This is where I’d just have to let go of making sure everyone can eat everything. They just can’t. I’ve tried many variations on gluten free (bread) stuffing, and they just don’t work. If I have room, I make a nice rice/wild rice stuffing for the gluten intolerant, but that moist delicacy that is bread stuffing? Sorry, Celiacs, it’s just not on your menu. Stuffing made with gluten free bread is a sticky glop with mealy bits there and soggy bits over here. Yes, even with Udi’s, the most amazing gluten free bread ever made. 
To be clear: My post was an illustration of a ‘traditional’ Thanksgiving spread; not a suggestion that the OP should prepare everything mentioned. The OP’s menu sounds find for a Thanksgiving-themed dinner (though I’d suggest mashed potatoes instead of roasted ones).
About the canned cranberry sauce vs. make-your-own -
It is rather like frozen orange juice vs. fresh squeezed. Most of us grew up drinking the frozen kind, and whatever you had as a kid is traditional.
My dear mother always baked bread and cookies, so Wonder Bread and Oreos were something of a treat.
I got better.
Regards,
Shodan
Another vote for moving it to Saturday, for the simple reason that that’s when the English"footy" is on! Watching American football is a pretty big part of the tradition here, at least for me and pretty much everyone I know. You at least have the games on in the background after watching the parade on TV, corny as it is, & everyone sits around boozing/eating/napping. (& Alot of high schools have games on Thanksgiving morning before people sit down to eat.)
It’s a great way to avoid talking to people you don’t really know that well or like that much (ie: family )
Footy matches seem to start around three I think?, which is really a perfect time to eat after drinks, appetizers etc.
I think to really get the feel of it you’d want it to be more of an afternoon/daylong kinda thing.
Have fun!