Hi,
The rules and regs are as stated; basically 100hrs p1 and 60 hrs on micro etc. The 5 hours on type for instructor's course seems to be optional looking at a few logbooks I have seen!
Medical, it is just the standard NPPL medical.
As for costs: about 3k for the course.
The biggest problem in some parts of Britain is to find a place as an AFI for the next 10 months and 100hrs minimum. And to make sure that you do some real training and not just trial flights. I know of one part-time instructor in Scotland who was refused an upgrade to FI because most of his hours were TFs.
As for pay, once you are an FI, expect 20-25 quid an hour (although I pay 35) on a school machine. Of course, you could always find a location and set up on your own - no need even to register with the CAA as with light aircraft.
As an AFI, in some parts of UK you will get paid peanuts, literally. You need the school more than they need you, usually.
Very best wishes,
Colin