London pals, I need your help!

Hey everybody!

Soon, I'll visit London and stay there for 2 weeks. Since I've started using this app, I have shed a few pounds already and now I'm a bit scared to re-gain that weight, which is usually the case when you travel to a foreign country and encounter all the delicious foods it has to offer. So, I wish to ask, which healthy food shops/(super)markets can you recommend? I won't be eating in restaurants on a daily basis, because I don't have enough money for it, so I'm more interested in shops, really. Please keep in mind that I'm a student and can't afford to sell a liver just for a super-healthy meal

For example, last time I visited London, I quite often used to eat at Prêt A Manger, but I don't remember how healthy that option is?

I appreciate your help!

Cheers, Sasha