i think you could get onto a computer science course with those a-levels; most have said maths is actually more useful than computer science at a-level if you're planning to study it at uni. i've only looked at a few unis, but cambridge for example, all colleges have said that they want an a-level in maths, some asking for a science a-level too.
of course, the fact that you do coding as a hobby is likely to be very helpful also.