well first of all congratulations on getting into UCL. It was my first choice but I didn't get in (I ended up going to Durham and loved every minute of it anyway). And in the end I moved to London anyway to work, there is no place like it.
I did gain weight during uni and this was because of going out. I'm a good cook and I always eat healthily but I would go out drinking and grab takeaways/go to restaurants because of the social aspect- I found I couldn't avoid it but when I started I wasn't bothered about calories at all so I didn't make a conscious effort.
Be prepared for a challenge. It's so difficult to be restrained while meeting new people and trying to have a social life and even now I still sometimes struggle. In the UK, drinking age is 18 (obviously you know that) and student culture centres around going out, getting drunk and dancing. My advice would be to memorise how many calories are in alcohol (for me it acts as a deterrent) and, after said night out it's customary to head to the local greasy takeaway. Do not under any circumstances order anything at a the places that are still open after a night out. The food at those places contain an obscene amount of calories and aren't worth it!
One thing that will work to your advantage is that London is very expensive for night life (and accommodation and food), so stick to a budget (It will also cut down on calories through alcohol). Meat products in supermarkets are so much more expensive than in other parts of the UK, But Green grocers are everywhere and such good value for money. I do a week's worth of veg shopping for 2 people for about £6 (And I eat a lot of veg). There's also a weekly farmers market near tottenham court road (Berwick street) which you should check out (avoid the fried food they sell though).
In terms of going out for meals, you can eat healthily in London and still get delicious food so you can have the best of both worlds. There's a lot of restaurants popping up that emphasise healthy eating (and a lot of restaurants have their nutritional information on their website).
Soho is the best area for restaurants, you have a fantastic mix of cuisines and if you stray off of the beaten track you'll find some amazing places to eat. I especially recommend BiBimBap on Greek street (http://bibimbapsoho.com/
) I can eat there on my calorie budget, the food is basically rice, veg and topping- they do tofu which is very good for you and low calorie and you can opt for brown rice served in a stone bowl and it is delicious.
If you stray into Camden (I recommend you do!) There's a Japanese place in Mornington Crescent called Asakusa (265 Eversholt street)- You HAVE to book though, it's cheap and very popular. They do great grilled meat dishes with stir fry veg, miso soup and rice.
Vietnamese food is also very healthy and usually salad based, there are several great independent chains dotted about in Soho.
Hummus Bros (Wardour Street) is sooo good, healthy and they list their cals on their website.
Err... if you just fancy a sandwich I recommend Pret a manager because all calories are listed on the price labels so it's basically no fuss eating. They regularly do soup, some of which is really low calorie and I lived off of when I first started working.
I hope this sort of helps as a mini-informal guide to London.
Edit: I forgot, Nosebear.tumblr.com is a great food blog set up by a student who's been travelling. The recipes are easy, cheap and there are some real low cal gems in there. Some of them call for sugar (but I usually sub that for honey or just omit completely and it still tastes really great). Try Swahili chicken, jamaican stew and chilli recipes!