From the FAQ on website link above,
Q: I live in the UK and have an FAA private license and instrument rating. I understand that the UK CAA will give me a UK IMC rating on that basis. Is this true?
A: That is an old wives tale that has been circulating for longer then the 28 years that I have lived in the UK. I have never met any one that actually got their IMC rating that way. As the CAA can't issue ratings on foreign licenses, you would have to do at least three written exams for the UK PPL, the written for the IMC rating, training with a UK instructor as well as the skill tests for both.
I know a couple of people who have obtained an IMC this way, just by writing in, no exams requried!
J.