Computer Science is a great subject in its own right – it gives you practical skills but also allows you to develop an understanding of the role and impact of computing in the world. As such, it can really go with any combination of subjects as the transferable skills you will develop will be useful in any career or path of study. Those students wishing to pursue a Computer Science degree should combine it with A Level Maths as this is a pre-requisite at some universities.
In terms of subjects that go particularly well with Computer Science, Maths is a very popular option as much of the A2 theory involves studying the decision logic. Physics and Psychology are interesting combinations; particularly with the increased dependence that we have on computers in our lives and the rate at which artificial intelligence is progressing.
Maths, Economics and the Sciences are good choices to study alongside Computer Science as the practical skills developed will prove most useful.