I suggest that it is a bit too soon to decide on which universities to apply to. Your academic interests may develop and change between now and the autumn of 2026, when you would be making your applications with a view to starting your degree in 2027. In addition, which universities you apply to will be influenced by how you do in your GCSEs, and how you are predicted to do in your A levels or IB.
Also, please don't fixate on Oxford or Cambridge. It is great to aim high, and best of luck in doing so, but most applicants for undergraduate courses in all subjects at those two universities don't receive offers. An application to either should always be seen as a long shot, and rejection is not failure. Please look also at the several other good universities in the UK.
In the meantime, if you get the chance visit both Oxford and Cambridge to form an impression of the two places, have a look at the Oxford and Cambridge Demystified threads here, check the universities' websites for information about courses (this changes from time to time), and look at the alternative prospectuses for both universities, produced by people who study there.