FFF,
Not according to Simon White of CAA Licencing. If you hold a ICAO Licence with an instrument rating then you are entitled to exercise the privledges of an IMC Rating whilst in command of a 'G' Reg aircraft - even if you have not sent away for your IMC Rating to be added. It doesnt matter. You have exactly the same priveldges as if you had sent the £64 off, but you are in pocket and no hassle as its valid as long as your IR is.
He was questioned by a JAA/FAA Instructor friend of mine, with specific mention of the FAA IR.
Julian.