See the following section in LASORS A8:
Procedures for holders of JAR-FCL pilot’s licences to transfer state of licence issue to the United Kingdom
Note also the following from LASORS E3.1:
The IMC Rating is a national rating and can only be
endorsed onto a UK or JAR-FCL aeroplane licence.
The CAA will only add an IMC Rating to a UK issued
pilot licence. When you are training for the issue of a UK
Instrument Meteorological Conditions (IMC) you must
follow a syllabus recognised by the Civil Aviation Authority.
Training for the IMC rating is permitted outside the UK in a
non-JAA state provided that the flying training is conducted
by an organisation that is fully approved by the UK Civil
Aviation Authority for such purposes. Instruction on the
course may only be given by an IRI or a flying instructor
who is qualified to teach applied instrument flying.