I can answer half of that Bob. It's a nominal fee (I think £42 last I looked) and a form to get issued a parallel UK CPL. You can then run the two together, and just ask examiners to sign both after you have done rating revalidations. The UK CPL will not list the IR(R) as it's considered embedded - and the currency requirements seem to have been addressed above. For now at least you can use a UK licence to fly EASA aeroplanes, and for non-EASA aeroplanes (such as the Bulldog I mentioned up the top) that privilege should never expire.
Somebody who knows more about that (and your own qualifications) than me can answer the bit about training and testing.
G