As far as I understand it,
To earn as an instructor you need to hold a CPL, 200 hours (including 150 PIC)and have completed an FI's course. This will put you on the very bottom rung of the ladder. No doubt a quick search of this site would reveal many more detailed answers.
It is possible to instruct on a PPL (not for remuneration of course) but I think the hours required increase to 300 and obviously you would not need to hold a CPL.
The details can be got through the CAA website LASORS.
The route I am taking is:
PPL £4500
Night Rating £500
Hours building £6500
CPL/ATPL distance learning £2000
CPL £4000
FI Course £5500
To the grand total of £23000
Gulp
Obs cop