The CAA website says that if you have a CAA PPl with IMC, you are exempt 12 hours from the JAR IR course, It is not clear if this applies to an IMC on a JAR PPL or CPL.
The IMC was intended as an add on to the PPL, if you intend doing a full IR, the cost of 15 hours plus test will probably cost more than the 12 hours it will save you from the IR.