2close, Is this correct?
It is my understanding, as you quite rightly point out that you cannot simply pass CV tests in another country and then ask for the CAA to take the results of these into account for a UK CAA Class 1.
However I also understand that you do not need to have say a German (or any other JAA member state) CPL/ATPL before the UK CAA will convert a full class 1 medical in another member state to a UK one.
So, if I haven't confused you already as I understand it you are able to convert a JAR medical to a UK one as an administrative exercise without having to have completed CPL / ATPL studies in that JAR country.
As always I stand to be corrected.