If you're doing Maths, I would stick with Physics. Some of the Physics content is also covered in Maths (such as mechanics), however it's all down to what you want to study at uni (presuming you want to go to uni?).
Also, just because you were stronger at one subject as GCSE than another, does not automatically mean you will be stronger at that particular subject than the other at A-Level. A-Levels are a completely different ball-game. My strongest subject at GCSE was Biology by a clear mile, and is my weakest currently at A-Level.
Good luck!