It depends on whether or not you stay at your school's sixth form or got a sixth form college. Generally speaking, sixth form is much more sociable and involves a lot of independent study as it's a lot more detail (but I'm assuming you gathered this already). I'm at my school's sixth form so I don't know what sixth form college is like but it should be pretty similar either way; it's just at school, you obviously still have to wear uniform, formally address your teachers, follow school rules etc (but then again, sixth form colleges would have rules, too). I'd advise you to choose the A-Levels that you're actually prepared to work for, regardless of whether or not they're STEM. If you genuinely enjoy the subject and feel willing to put the work in, take whatever you feel passionate in. It's better to get A*/A at a non-stem subject than a C/D in a STEM subject that you took just for the sake of it being STEM.