I have no idea if British high school s have lockers. I assume you're talking about high school, right? Linz... help out here?
As for IBS, yeah, I had it while I was there. I lived in a self catering flat though (that is, it had a kitchen and I was responsible for purchasing and making my own food) so it wasn't an issue. I lived in an entirely vegetarian flat, and I knew quite a few vegans, so you should be able to eat just fine.