If you get your IMCR done before July 2012, it's yours "for ever". It will be added to a license as an IR (restricted). Being limited to Annex 2 aircraft (most of which can't fly IFR) it is a bit useless but the CAA said they are working on sorting this out with EASA...
Slightly fuller report:
The intention is that an IMC remains valid on a UK licence, but only for Annexe II aircraft. The man from the CAA said that anyone with an IMC rating pre-dating July 2012 would, when issued with an EASA licence, have an IR(restricted) tacked onto it enabling IMC rating privileges in UK airspace
in EASA aircraft.
All subject to approval from an EASA standardisation committee, but the CAA guy seemed fairly optimistic.