Not sure if you’ve seen http://www.nationalrail.co.uk/, which must be the most popular way of checking schedules / ticket prices. It does only go three months ahead, but schedules and prices shouldn’t change much between now and July. It looks like off-peak return tickets to London start at £20.
Your visit isn’t in the football season, but pre-season friendlies will have started by then. The atmosphere won’t be the same, but if by any chance Bristol City arrange a “glamour” friendly, it might be worth catching. Keep an eye on http://www.bcfc.co.uk for fixture announcements.
I wouldn’t worry about buying a shirt just to go to a game, but if you do want to buy one as a souvenir, I find the sizes very variable and always check before buying. You might find an XXL is the same as the US, you might need to go up to an XXXL.
For some of your other questions re terminology, don’t worry too much, you’ll manage to make yourself understood. We have lots of US TV on rotation over here so it’s not totally unfamiliar. The only one I’ve seen raise a quizzical eye is asking for the “rest rooms” in a pub or restaurant. (Doesn’t seem to come up too much on “Friends”…). Ask for “Gents” and you’ll be fine. (And yes, I know that’s a whole other thread, let’s not go there).
You can also use www.thetrainline.com to book tickets in advance, very cheap.
Yes everyone takes Visa - what do you think this is? Somalia??? 
Well as a London dweller I think a weekend trip here would be worth it and would be happy to give you some tips on getting the most out of a short visit, but I agree with the others that you could equally just stick to Bristol and Bath and have a lovely time.