OP, I realise that the U.S. doesn't like the U.K. much... it was something about a tea party as I recall?
Jokes aside, the bad reputation of U.K. food in the U.S. comes from U.S. troops stationed in the U.K. towards the end of WW2 when there was strict rationing, most of the U.K. had the hell bombed out of it and there was very little food and most of that was canned since there wasn't enough gas to get regular shipments in from the countryside. If I visited New York the day after 9/11 and then spent the next 50 years complaining about how U.S. food sucked because I couldn't get any decent food and half the restaurants were closed... well, people would be right to call me an insensitive prat.
A lot of the food you eat in the U.S. is from the U.K. Of course it is hard to draw a clear line between U.K. food and European food in general, but U.S. food culture is from the same roots. The hamburgers people in the U.S. like? They're from Hamburg. Those hotdogs that are regular additions to the U.S. lunch table? They're based on German food. Most of the stuff you eat during Thanksgiving is pretty much standard food in a traditional Sunday lunch in the U.K., although ingredients may differ the cooking style is almost identical (e.g. U.S. Roast Turkey, U.K. Roast Chicken, Duck or Gooose, U.S. pumpkin mash, U.K. mashed potatos or other mashed yams, etc.).
As a result I find it quite amusing every time someone from the U.S. criticises U.K. food... in their ignorance not realising that there's about a 90% overlap.
Sure you'll find some revolting food the U.K., soggy fish and chips, pies filled with miscellaneous bits of gristle, etc. ... just like you'll find some utterly revolting food in the U.S., like hotdogs that have you throwing up inside of 5 minutes (and that the locals claim are utterly delicious!!?!) or suspect falafels that contain more rat than lamb (I'm thinking of a particular place in New Jersey).
What do I recommend? Go to any decent pub (bar) and have a "pub lunch". You'll be pleasantly surprised and generally they're reasonably priced since the pubs make most of their money off the booze. If your hotel offers a buffet any of the evenings you're there then I recommend trying a bit of everything. The quality isn't normally amazing at buffets, but you'll get to try a good range of dishes and flavours for quite cheap. The pub meat pies are normally quite good.
Obviously if you're rolling in money then I recommend Gauthier in Soho for a truly traditional dining experience. And don't be mislead by the names of the dishes being in French, most of the food is authentically British in character, but the French simply were the first to come up with a system for naming how stuff is cooked, which the British adopted. Shetland langoustine and scallop is amazing, but so is the Tornedos of Angus beef (... yes, I know there are Angus cows in the U.S., no they do not come from the U.S, they're originally Scottish... and delicious). A dinner at Gauthier will set you back easily 100 pounds, if not more, but for a truly British dining experience I recommend it.
What to avoid? Anything that's a chain store. You don't call McDonalds American cuisine do you? No, I thought not. Try to avoid the same mistake with British food. There are some good chain stores, like Nandos (best darned chicken in the country!... but not very British), but as a rule they're lousy. Likewise anything you buy from a guy on the side of the street you're taking a risk - sometimes it is amazing... and then you can never find the bugger again, and sometimes it is complete garbage and you're bent over a toilet for 2 days. Basically follow common sense like you would if you were visiting any city in the U.S. Don't assume that all food in the U.S. is lousy simply because you had the bad sense to buy a rat and cheese falafel from a guy on the side road with a cart in New Jersey.
