need a level advice!

I really would like to do Maths at a good uni like Oxford or Imperial, but I'm stuck for a level choices.

Would I be able to get in to a good course if I chose Maths, Further Maths, Computer Science, and Economics, or do I need to take another science like Physics or Chem?

I would have a look at the entry requirements for the courses you think you might want to study at university on their websites, this will give you the most info. If you want to study Maths at a top uni, a few require further maths and definately A-Level maths, however some only require A-Level Maths. If it is definately maths you want to study I am pretty sure that none of the unis require you to take a science at a level enless you want to do medicine, engineering etc.

If you're set on maths then those choices sound good.

I think a lot of people also do physics/chem because at uni those subjects become essentially just applied maths as well, whilst maths becomes weird conceptual proofs ad infinitum, and a lot of people are actually after the former.

I don't do maths (more of a social sciences type student), however, something I will always recommend is to study something that you and enjoy and are good at, and therefore you will do well! It's much better to do well than to do ok in subjects that you think the universities want. Oxford and Imperial will be much more impressed if you get an A* in economics rather than an A in physics. If its not compulsory, don't worry!