Imo you should take Maths, Further Maths (if you're up to it) and Physics and/or Chemistry. They keep your options very open and are highly regarded A-levels in STEM irrespective of which area you go into. Those A-levels could get you anywhere from Med school to an aerospace engineering course.
If it turns out you really enjoy computer science and want to study it at uni, you will not be disadvantaged by not having taken computer science at A-level. I say this as someone who is applying for computer science and takes the subject at A-level. All unis are aware that most colleges/sixth forms do not offer cs, and even if they do the teachers might not be very good, so do not care much about whether or not you studied the subject. They do, however, care a lot about maths and further maths if you can study it.