jamup - the words 'cheap' and 'JAR' rarely appear together!
First you'll need a JAR-FCL CPL(A) if you want to be paid as a Flight Instructor. That will be quite expensive and you'll have to sit a fair few exams as well; most people don't bother with the CPL level exams, they take the ATPL(A) exams and achieve a CPL(A) with ATPL knowledge. But the ATPL theory credit dies if you don't also obtain a JAR-FCL IR(A) within 36 months of passing the exams....
Once you've obtained a CPL, you'll have to attend a JAR-FCL Flight Instructor course as described in this thread. But should you wish to teach on ME aircraft, things become even more expensive. There is far less demand in the UK now for ME ratings except for a few private owners and the training schools. The old 'Group B' PPL was much cheaper to maintain, a PPL with MEP Class Rating is far more expensive to maintain.
Don't forget that there is currently a surfeit of FIs in the UK looking for work, so even when you've achieved all your licences and ratings, you'll have to join the queue........