Depends.
Supermarkets pay well and seem to pay around £10per hour now after annintitial probationary period and pay everyone the same regardless of age. I think McDondalds is same.
Many non chain shops seem to pay £5-6 per hour.
People doing stuff they have to be qualified for often get more…sports coaches, lifeguards, swim teachers etc. If you can do a course to qualify in one of these, you can earn better money.
Working for one of the group tutor firms like Kumon seems to pay reasonably.
Some companies are more flexible on hours than others. Supermarkets often want 13+ hours and some want 20+ which is probably too much for A Level student. Some might be zero hour which is flexible but often means no work or they want you to always be available and to never have to give you any work.