I want to take a Computer Science degree - should I take a philosophy A level?

I'll be completely honest, the most important A-levels for computing is maths and further maths. Physics is also very useful, even more useful than computer science, as it allows you to apply even more maths. Maths is key. Unis such as ICL, Cambridge don't mind if you don't have computer science. For your 4th choice, I'd recommend choosing which subject you'd enjoy the most, but physics is more useful for computer science degree than philosophy.

I'll be completely honest, the most important A-levels for computing is maths and further maths. Physics is also very useful, even more useful than computer science, as it allows you to apply even more maths. Maths is key. Unis such as ICL, Cambridge don't mind if you don't have computer science. For your 4th choice, I'd recommend choosing which subject you'd enjoy the most, but physics is more useful for computer science degree than philosophy.

A-Level Chemistry or A-Level Computer Science as a 4th subject

Which is better for Oxbridge, ICL etc?

Both are great. A thing to note is that Chemistry is a lot harder than computer science. For computer science, it doesn't really matter, and I'd recommend choosing what you enjoy the most. Chemistry A-level would open up more doors in terms of degree options if you choose not to do computing (which I'm assuming you are)?