An IMC rating granted on the back of an FAA IR apparently does have to be revalidated after 2 years, regardless of the currency of the FAA IR.
That's because the IMC rating is a UK rating granted on a UK (or JAA) licence. The FAA IR serves only to reduce or eliminate the training or examination requirements. Once obtained its existence is independent of the FAA. If the FAA IR lapses in currency, you still have an IMC rating.
If the FAA IR remains current and the IMC rating lapses, the FAA is still rendered valid by Art 21(4) which permits its use on G-reg outside controlled airspace.
The oddity, I guess, is that if it lapses you cannot tear up your old IMC rating, write the CAA a cheque and apply for a new one on the basis of the current FAA IR. Or can you?
