You book and pay yourself. Best do this before any thing else because if you fail the medical then there's no point in going on until the issue's resolved, if it can be.
If you want one, you should organise it too. See GAPAN etc. Some schools do them, ask first.
Maths and Physics.
I crashed and burned them at school. It's basic.
CPL v IR
CPL then M/E IR is the best option but it may not be in the UK. My reasoning is to get all the VFR stuff out of the way before tackling the IR stuff. I don't know what the benefits are hours wise in the UK.
For some reason JAA land deems the IR (a rating) more important than the CPL (a license)...