pretty much any supermarket, shop, cafe, mcdonalds (if your grades aren't too high), call centre
supermarkets may not be your best bet as they often want people to be prepared to work unsociable hours which you probably can't do (unless you drive?)
personally I worked in a call centre during my A levels and it was a pretty good job, I earned maybe £55 a week on average for 9 hours work, and I did those hours in short shifts after my college day finished and still had my weekend to myself