For the UK, stick in at both computing and maths. To get into university courses for it you need at least the top level of GCSE for both at minimal entry (if you dont get them, you can still get in if you go for your A-levels).
I don't know about England, but up here in Scotland the University of Abertay is considered the best one to go to.
No matter where you go, the games development industry is insanely competative because its a new industry, everyone wants to do it and most of the people with jobs in it are young so wont retire or kick the bucket soon. There are more gamedev graduates than jobs out there.
So on top of a qualification you will need a portfolio of projects as the edge to get you in there. So to be frank the sooner you pick up C++ and start making small games the better.
Try and learn as much as you can out of class and do examples out of books, remake old 2d games to cement what you have learned. Don't bother with 3d until they teach it at university or you will just get overwhelmed.
