MEP rating is a class of aircraft, ME is a subset of the IR rating.
You can be a IR instructor on the F406 which is ME but you would need to be a CRI on that type as well. But unless you were a CRE on MEP you couldn't teach IR's on it.
It all gets a bit involved but to teach IR on piston aircraft single engine.
You need.
SEP class rating
SPA-SE-IR.
IRI-SE.
or in the UK FI with applied instruments removed.
For MEP
MEP class rating
CRI MEP
SPA-ME-IR
IRI-ME
CRI on the class or type your teaching on
or UK FI with MEP and applied instrument restriction removed.
Pre-Course Entry Requirements
An applicant for an IRI(A) certificate shall:
(a) hold at least the licence and the rating for which flight instruction is to be given
(b) be entitled to act as PIC on the aircraft during flight instruction
(c) have completed at least 800 hours of flight time under IFR, of which at least 400 hours shall be in aeroplanes
That's what the current pre course experience levels required. There is no way in hell that any single engine PPL instructor will be able to get 800 hours IFR time.
plight of many pilots across Europe who all want more accessible qualifications for instrument flying
Even if you get the qualification there isn't going to be anyone to teach it. The teaching IMC was within the grasp of most PPL instructors and in fact before 2000 it was a requirement to become unrestricted. Which was why the IMC rating had such a huge uptake as people very easily could go PPL, night then IMC.