Hi.
Well let's look at the A Levels you need first, then try and backtrack to see what you need to do for GCSE.
A Levels some good unis would like is Maths, the top end, if Oxford/Cambridge/Imperial etc is your goal, will probably be Further Maths too. A good proportion of CS applicants take Physics, now whilst I'm unsure if this is inherently a requirement, it is quite liked. The 4th subject can really be whatever, some take CS A Level, some take Chemistry, etc. Maths, FM is the key, and Physics if you want to boost your odds.
Now, you think, why is CS A Level not mandatory?
It's not really mandatory for many unis, infact, and some don't inherently take to it. This is because CS A Level has immense breadth but not much depth, so really it's just a foundation course for degree level. However it won't really REDUCE your odds, so if you're truly passionate about CS, and feel you'd do best in that A Level, take that, as long as you have Maths, FM, and maybe Physics,it really doesn't matter.
Now, how can we get to that path by GCSEs?
Firstly, Maths and FM will just require grade 7-9 (depending on your school), in Maths. Physics will be about grade 6-8 depending on school, and most of the sciences are, so your first priority is to attain in your core subs, esp Maths, and the sciences.
Options?
Well if you're intent on doing CS A Level, CS GCSE is a good start, I did it and it was fairly fun. However do note that some schools don't actually require CS GCSE for CS A Level, and just required a grade 6-7 minimum at GCSE Maths, check up on your school.
Besides that?
Well, it really doesn't matter! Unless you're intent on picking a subject at A Level like Geography or French, the GCSE options don't matter, of course since CS A Level isn't mandatory and if you'd prefer something else at A Level, you'll have to attain in that too, but don't worry too much about your GCSEs for CS.
Oxford?
Well they don't have set requirements in place for A*s, but they'll want the applicants to have majority grade 7-9s in their results.