Food is alright at halls, but best to get a group of mates and get a takeaway together (pizza, chicken, ect).
Bankside is usually the most social of all LSE residences, got a good common room and bar. It's one of the biggest halls in london. And there's loads of events throughout the year (right from the begining).
You'll find out about food when you get to London, depends on your taste, best to find as many deals as possible, and use them all the time. There's no point of goin to expensive places, you can use that money on booze instead hahaha..
I don't think many students know what breakfast is??.. I don't??!!
Supermarket.. hmm.. you really do love ur food haha.. Google is ur frend!! Again depends on where you like shopping, M&S and costcutters are the closest tbh.
Lunch, eat on campus or subway type shops.
Hope that helps.