There is the ticket for the bare travel, then there is the reservation (plus additional money) if, for example, you want first class. We did this four years ago, so there are details I don’t remember, but that’s the basic thing.
One thing is that now is a good time because the dollar is relatively strong against the euro, almost 1 to 1. It was about 1.35 to 1 I think when we went.
The trains themselves are great. First class is great, as you might expect, but the next class down (2nd? not sure) is no slouch either. I’m not sure what the lowest available class is, we never looked at using it.
Some things about the departing station (we departed on a train from Paris, but we didn’t hit London so that might be different): you have to wait in a large waiting area serving a lot of platforms until maybe 30 minutes before the train leaves; then they allow you to go to the platform. Places to sit are not plentiful, but if I recall there is at least one café you can go to sit down and snack on something, on the second level, that overlooks the waiting area. There is a big board where all the upcoming trains are listed, and this is where you find out what platform your train is at. I didn’t like this part much, I was always afraid I was going to do something wrong and miss the train, but I was fine and we always made it with plenty of time to spare. The walk to the car where your seat is can be kind of long, depending on how far down the train you are.
Now some things, very important, about arrival stations. When you arrive in Paris and in Rome, be sure to have the name and address of your hotel (or wherever you are going first) written on a piece of paper that you can hand the driver. It avoids misunderstandings because of your foreign accent. Find the taxi queue, it will probably be quite long but it should move fast, there were tons of taxis waiting when we went, and wait quietly in the queue until you get to the head of the line, and let the guy in charge get you a taxi. Do not allow anyone to tempt you to depart from the line and try to give you a ride. At best they will be unlicensed and you will get yelled at by the licensed cabbies. This happened to us in Rome (what did we know?) and a fierce lady cabdriver drove off the interloper, and during the entire ride she yelled about what had happened as if it was our fault (all in Italian). I tried to make allowances, but that was very unpleasant.
The trains, by contrast, were always pleasant, extremely clean and comfortable, and fast (you can take a slower train if you want, I guess, I didn’t really check out that option). We always traveled in the daytime so we could see the scenery. The farthest single leg we traveled was from Venice to Naples, and I think it was 5 or 6 hours. I don’t know about London to Paris (Google Maps says 3 to 4 hours), but Paris to Rome will be more than that, over 10 hours if you’re not stopping anywhere. I presume they have sleepers if you want to travel at night, but again, we didn’t do that.
I can’t give you much advice about navigating the website, I managed it so it can’t be too difficult, but I don’t remember much about the experience. We got Eurail passes, which saved some money, I don’t know if it always saves money, it probably depends on factors like how long you will be there and how many different stops you make. One thing to be careful of, if you get Eurail passes, they have to be checked off exactly in the right order by each conductor (I had had no idea about that, we missed the first one somehow but the next conductor graciously forgave us). I think this is so that people don’t try to cheat with the passes. Just read all that stuff carefully, and make sure your first conductor sees your passes. I don’t know if Eurail passes cover London-Paris, if not the passes may not save you money.
This was the first time I had been to Europe and it was simultaneously exhilarating and sort of terrifying and aggravating. Our taxi driver in Naples deliberately took us to a place 1.5 km from our hotel and left us there, with no other taxis in sight, and I never figured out why. It rankles to this day. I hated Naples. I’m glad you’re not going there.
Sorry if this is a little disjointed. Ask questions if you want and I’ll try to answer.