Math for Computer Science

If the course doesn't specify A-level Maths is required in its entry criteria then just GCSE is fine, although you will probably then cover some A-level content in the degree itself at some point (probably in first year). If A-level Maths is normally a requirement for the course you may need to do a bit more work to catch up.

I would probably recommend focusing on any algebra, functions, vectors, and graphs content from GCSE at the least, as I imagine these are the areas most likely to be built on. It might be worth looking at some of the AS level topics on those areas as well if you have time and are confident in the GCSE topics, even if A-level Maths isn't required for the course, as you'll probably do some work in those areas anyway.