As said above, I recommend / you do GCSE Physics Part-time / full-time (your school will probally let you sit it whilst doing as/a-levels)
Unfortunately docash isn't exactly right, you do need the 5 GCSE's including Maths but as you are well aware...also including the Physics.
I agree with the A-level comments, subjects are irrelevant really as they look at your foundation stuff, which would be your GCSE's.
I have first hand knowledge of this, I have 3 A-levels, 3 AS-Levels (Not Physics related) I also have a good Computer Science Degree BUT ! I do not have GCSE Physics.
Because of that I am required to resit my Physics at GCSE Level + before they will let me pass to Stage 2, rather annoying as you can imagine and believe it or not hard to do whilst not at school
You are in the best place to do Physics now. Speak with your school, tell them you should of done it etc and you would like to do it in your first year of Sixth Form / College, they will be more then happy to help.
Good luck.