Go into the Independent sector. Tell them what you want to teach. They will let you. You may do all three sciences with year 7's, but this will be fun. There is no physics + maths PGCE. A physics PGCE will give you more job options. You will be more likely to get a maths job with a physics PGCE than vice versa. Contact the IoP (better still, join!). They may be able to fix you up with a visit to an Independent school locally to chat about it. Failing that, just phone up local schools and ask to speak to the Head of Physics.