Perhaps careless wording on my part - the UK IMC rating is valid in CI airspace. However I have it in writing from the Manager ATC Guernsey that you may not fly a N Reg aircraft on a JAA licence in the CI zone. I don't believe this has changed under the shortly to be introduced revised ANO - but I will check this and give you a reference.
Can you fly a JAA registered aircraft in the CI zone, and use IMC rating privileges? I believe so.