(Original post by FDR)
Physics is a very good A level to have, but as you're also doing Chemistry, Maths and Biology, perhaps taking a social science or humanity (Economics is good - no course-work to worry about) would make you appear as a more well rounded candidate.
If you do decide to do Physics, I'm currently doing A2 Physics with Edexcel, so I'll give you my take - AS level Physics isn't too difficult, the exams are out of 80 marks each, with 10 pretty easy multiple choice (although one or two can be horrendous), with the rest a mix of calculation questions (at AS, the most advanced maths you'll do is a bit of simple trig, but mostly just re-arranging and substituting numbers into equations) and short answer responses. The topics vary in difficulty - Kinematics and Newton's laws are simple, but Capacitance and Electromagnetic induction can take some getting your head round.
As for the exam board, all I can say is that Edexcel is awful. You get given the mark scheme when you do coursework, yet getting the marks can be quite tricky. Plus I find that in the exams they try to put it in context too much, and this just makes questions complicated and harder to understand. I'm also doing Maths and Economics at A2, and I'd say it's my hardest A level.