tim,
you are almost right. if you hold a multi pilot IR, then you are deemed to have an implied UK IMC rating. that is, you can exercise the privilidges of an IMC rating but not an IR rating (cos you don't hold a Single Pilot IR).
there is not much difference between the IMC and IR. can't go in class A, take off minimum of 1800m and there are advisory additions to instrument approaches.
also, this is only valid in the UK airspace. many a IMC pilot has flown the ILS at Le Touquet only to find the DGAC at the end with a 1000 franc fine.