The refresher training required before renewing the MEP class rating is entirely at the discretion of the Head of Training of the ATO. There are suggested figures in the AMC that the ATO
may take into account but it is not obliged to do so. If you are current on MEP under another authority, I would not expect much if any refresher flying to be required. Try dropping a PM to Bose-X on this forum.
As to the IR, the following appears in the new edition of CAP 804, published yesterday:
The UK CAA has issued an exemption such that where a pilot holds or has held an
Instrument Rating issued by a third country and that rating is compliant with Annex I to
the Convention on International Civil Aviation, the applicability of FCL.625 IR(c) and (d)
may be based on the validity dates of the Instrument Rating of that other country. The
effect of this exemption is that to renew the IR on a UK issued licence:
(i) a pilot with a current and valid 3rd country IR shall complete the revalidation
requirements of FCL.625(b) and the aircraft category specific requirements for
revalidation of the Part-FCL IR; meaning that he must pass the proficiency check,
but is not required to undergo training or to re-take the theoretical knowledge
examinations; or
(ii) a pilot who held a 3rd country IR that is no longer valid but had been revalidated
or renewed within the preceding 7 years shall comply with the renewal
requirements of FCL.625 IR(c), but is not required to re-take the theoretical
knowledge examinations.
Official Record Series 4, No.974 which contains the exemption is
here