I'd say it's definitely worth looking ahead. I think there are a number of things you should consider including location, course content and modules, student satisfaction, rep of the uni and grades required. I've just finished my AS levels and am going to apply for music at Oxford, King's College London, maybe Manchester, as well as RWCMD, trinity, and maybe RNCM.
I think it's really important to decide whether you want the emphasis to be on the academic study of music or practical music performance. In my opinion Oxford offers greater performance opportunities in the modules you can choose throughout the course than Cambridge. In addition, many oxbridge music students get instrumental lessons at conservatoires.
Many music applicants apply through UCAS where you have 5 choices, and CUKAS where you have 6, and see where you get offers from!
Definitely go to the places and look around, read prospectuses, go to open days. Joint course between RNCM and manchester looks crazy hard to get onto but you get the uni experience and academic degree, whilst also getting world class instrumental tuition.
One more thing to bear in mind is that music is a tough world to make money in... even if you don't think oxbridge is necessarily suited to you, having such a respected degree can only help your career prospects. If you CAN get in, then by all means do it! Many graduates go on to do post grad courses at conservatoires, or get great jobs. But don't do something you won't enjoy...