You could check out restaurants online, and have an idea of some of the calories contents of stuff you might like to eat. Honestly, though, I'd say it's only a couple of days. Just enjoy yourself - don't go stuffing yourself silly, as you WONT enjoy that, but don't spend the whole time thinking about your weight.
The most important thing you can do when you're away, to minimise any 'damage', is to drink, drink, drink, drink water! I had a weekend away recently which included 8 LOVELY meals out, loads of crap on the journey and 15 pints of Guinness/ale. I also drank as much water as I could (15 pints of beer one day - 20 pints of water the next!) and come WI I was only 1.4lbs up!
Life is for living - just don't do anything that's gonna make you sad in the long term x
Eating on the go, I tend to grab either a Subway (turkey and ham all salad, no cheese, low fat dressing on a 6" wheat) for approx 300cals, or sushi from Tesco or M&S, small packs only approx 220cals. Then dinners if you go for pub style meals, you could have steak & salad, jacket potatoes, pizza express do diet style pizzas with the middle cut out, all their cals are listed on their website. Um, what else? Take a bag with some low cal treats for you when the others are eating the naughty stuff, or remember some treats and their cal values so that if you grab something your not doing to much damage ie sml bag quavers only 88, a small twister ice lolly only 85 etc, better choices really.. Most importantly have fun
I agree with all of the above, but here are my suggestions for foodage places in London.
First of all my number one choice for healthy, organic, environmentally friendly food (with full nutritional info) is Pod. They're all over the place, though not as common as pret or starbucks or eat (though Eat is another good one). They're my absolute favourite - I always have one of their spirulina smoothies when I can (delicious!) they are good for breakfast (wholeheartedly recommend their scrambled eggs and tomatos - c'est magnifique) and lunch (try the thai green curry) but I think they mostly shut at 6. You can also try Hummous Bros - I've never seen any nutritional info for them, but they're super healthy.
I think you should have an evening meal where you don't have to worry about what's going in because at the end of the day you're on holiday and as long as you're good during the day, then you can afford to be more lenient during the evening. My absolute most favourite pub in London is the Cheshire Cheese on Fleet Street. Its this old pub which was one of the first to be rebuilt after the Great Fire of London in 1666 (it was rebuilt in 1667) and has been there ever since. Its seriously amazing and has sawdust on the floors and everything its brilliant. Its an old haunt of Charles Dickens and Samuel Johnson. The restaurant is family friendly, and while not the healthiest, is all handmade fresh and so you don't have to worry about nasty preservatives or hidden fat - its like you would cook at home from scratch. And really cost effective too! Yeah, can't recommend that place enough. Even if you don't eat there and just go for a quick drink.