Whether CPL or PPL, the pre-entry requirements for the FI course are the same (except some experience concession for CPL holders). I.e you will still have to do CPL written examinations, and unless you can pass the UK exams in their next and final sitting, this will mean enrolling on a course with an approved training provider.
There is some talk of a reduced requirement for instructing on the forthcoming national PPL, but nothing has been agreed as far as I know.
A Class 2 is sufficient for PPL/FI (unremunerated).