If all you have is a JAA PPL/FI then no matter what other ICAO licences you have, be it ICAO CPL/IR ME whatever you will *not* be able to be paid to instruct on British registered aeroplanes. You require a JAA CPL/FI to be paid to instruct.
But to answer your question, I am sure there would be a flight school out there willing to use you as an instructor, simply because they keep all of the revenue! But maybe only as part time or freelance...
As you have to do the CPL or ATPL exams to be eligible to take the FI course then you might as well obtain a JAA CPL to which you can attach the FI rating, its only a 25 hour course and if the person already has an ICAO CPL then the requirement is "training as required to pass the skills test", no minimums as such. This is of course assuming one can pass the JAA Class 1 Medical which CPL holders require.
You'd then be eligible to instruct and be paid at a flying club... more change of a full time (and being able to fund it) position then.