The legislation may well change in 2012 under EASA, but as it currently stands you can do a FI(A) on a PPL so long as you meet the hour requirements (200 PIC rings a bell) and do the CPL written exams, i think 7 in total.
You can then do the FI(A) course itself, which costs around £7k and instruct for free.
If you want to earn money from it you need a CPL, which is a 25 hour course in a complex aircraft costing somewhere around £10k+
All the exact hours can be found in LASORS, or the ANO.