fireflybob, you won't be able to instruct for the Part-FCL IR unless, in addition to holding a valid IR, you meet the requirements of FCL.905.FI(g), which includes the requirement for you to have completed as a student pilot the IRI training course and to have passed an assessment of competence for the IRI certificate.
It's not just a question of holding an FI certificate and an IR - there's more to it than that!
However, once you're suitably qualified under FCL.905.FI(g), I don't see why the UK CAA wouldn't accept that you may instruct for the IR(R).
(Incidentally, there are more than a few non-EASA aeroplanes which fly in IMC!!)