There are two separate issues here, just in case anyone is confused by the discussion.
If you apply to the CAA for an IMC rating on the basis of your FAA IR, you may well be granted one for no more than a payment (i.e. no test) and you then have an IMC rating which you have to keep current in the usual way.
If you do not do so, you can still exercise the privileges of your FAA IR on a G-reg aircraft but not IFR within controlled airspace (including classes D and E).
There is therefore an advantage in applying for the IMC rating in that you can then fly IFR in classes D and E.