I know from experience with the CAA that the MEP Rating is a requirement for the M/E IR course although I cannot find the specific regulation at the moment. I believe it is a requirement before you start the course so that you can learn on a M/E aeroplane (satisfying the 15 hour M/E minimum). The MEP rating should certainly not cost an extra £3000, more in the region of £1800 - £2000, or less if you combine it with your CPL course.