Presently, you need an FI rating to be able to teach people (£6k) and a Commercial license (£5k ish) to be paid for it. I think plans are a foot to scrap the CPL requirement due to a lack of Instructors nation wide so you might want to research that thoroughly before taking up the CPL.
If you want to make a career out of it then you would probably need the CPL ME/IR anyway as you can only teach a student a course that you have been through yourself, then teach at a commercial school, but only after you are unrestricted and meet the requirements.
Lasors07 will describe what you need much better than I can. A word of warning though, PPL instruction doesnt pay that well unless you get to DepCFI,CFI or CPL IR level.
Good luck with it all.