OK. If you hold a valid IAA class 1, you can approach the CAA (now operating to JAR), who will carry out an initial class 1 examination, but who may allow for some of the tests completed by the IAA, subject to the IAA providing the relevant paperwork ( e.g. ECG, Audiogram etc.). In effect you would be doing only a 'renewal' type examination.
If you are only considering your IAA class 1 now, you should note that the IAA are performing JAR medical examinations w.e.f 01st Nov - this JAR class 1 is already fully acceptable to the CAA,so you would not need to undergo any further medical examination.
Hope this helps...