The CAA reportedly gives you the IMCR based on an FAA IR only if you had an IPC in the last 2 years.
They will exempt you from the written exams if you have passed an ICAO IR within 5 years (or other IR test, like IPC). They will also exempt you from the flight test if you have passed either the ICAO IR flight test or an FAA IPC (and probably some other foreign equivalents) within the last two years. The date of the IMC rating will run for 24 months from the date of the IPC or checkride.