The IMC ratig is not valid without a Certificate of Test - ANO 2005 Article 31.
In order for your 12 month IR to lapse to a 25 month IMC rating, the rating will have to be in Part XII of your Licence and you will need a valid Certificate of Test; this could have been signed by the IRE/CRE when you completed the last IR test.
Without that, your IR does not lapse to an IMC rating.
If you hold a UK CPL or ATPL then the licence only prohibits IFR flight in Class A, B and C controlled airspace; therefore you do not need an IMC rating to fly IFR in Class E, F and G airspace.
If you hold a JAA licence regardless of type, you cannot fly IFR without either a valid IR or a UK IMC rating for which a Cof T is required.
http://www.caa.co.uk/docs/175/SECTION%20E.pdf