Uni preparation tips?

Maths and Physics A levels are adequate preparation for Physics at university. Maths and Chemistry A levels are adequate preparation for Chemistry at university. Aim for As/A*s; if that doesn't fully occupy you, try stuff like the Physics challenge, UK Chemistry olympiad, general further reading (not pop science; stuff that actually expands your A level education). These are just to add some gloss to your application if you're aiming for top courses, but if you are aiming for top courses I'd recommend you put some extra work in.