D-B: >>"I understand that a FAA IR holder, is automatically entitled to a UK IMC rating. But am I correct in assuming that he would therefore have to hold a CAA or JAR licence, to which the IMC rating could be attached?"
Affirm.
( But also it doesn't have to be FAA as the start point, I think anywhere in ICAO land could have an IMC in this way providing they also had a CAA or JAA PPL to house it.)