I am off to London myself in THREE DAYS!
We have a friend who lives there and he is a bit of a foodie, so we'll be in good shape as far as finding good food. My suggestions for anyone going to the UK would be to eat in a pub. Not a "faux" pub as our English friend calls them but the authentic pubs. Also, a great website to check out is called www.fancyapint.com
you can plug in the tube stop you are by and it will rate the pubs close to you. On this trip, I want to check out the Borough Market and eat somewhere on Brick Lane. We're also doing a swing up through Scotland - I'm still not sure if I'll try haggis though (whisky yes, haggis no).
If you are on hunt for cheap eats, I like the Pret a Manger chains, EAT, ASK, and if you have a fridge, go for a quick run through the Waitrose grocery store chain.
I found a lot of good advice as to places to eat from the chowhound website also.