I'm worried about a future application to a University,?

I am an astrophysicist, but I'm not overly familiar with the UK system. However, there are a number of schools with good physics and astrophysics programs in the UK, so don't limit yourself to only one school. Besides Oxford and Cambridge, other schools with good programs include UCL, Liverpool, Leicester, and many more. You'll do your PhD somewhere else. If nothing else, you can always go to a school in the US - there are far more of them here, and most of them offer physics.