Do you even like Physics or Chemistry?
I get the impression you dont really like Physics and want to do it only because you believe it will enahnce your chances for uni. I did AS Physics even though i disliked it at GCSE (i was useless at it, esp electricity!) due to timetable clashes. I found the course mainly boring, but challenging and dropped it after AS. During the course I questioned why the heck i took it in the first place; it was pretty hard to get motivated. If anything, the maths modules helped physics not the other way round. I dont see Physics helping that much for a maths degree, Chemistry is just as rigorous (i found it more interesting as well!), and F.Maths will be more than sufficient to get you in. Bear in mind most of the top uni's now want or prefer at least AS Further Maths. In the end yes i was glad to have done Physics, but it was a struggle at one point simply because I chose something I didnt enjoy.
You could self-study F.Maths, but dont your school offer an accelerated maths course? That would be best. F.Maths can be self studied but if you're doing something like the Edexcel syllabus, i'm not sure you'd be able to do the F.Maths modules without the normal Maths knowledge.
Incidentally I had a similar problem with clashes between F.Maths and Biology. I chose to self-study Maths in the corner of the classroom so that i'd get course funding, and completed that in a year. It was difficult at times but enjoyable and you need to be fairly well organised. I'd recommend self-study as a last resort only if you've exhausted ALL other options. The maths board on here are THE best resource BTW!