Why downgrade your licence? If you hang onto you UK ATPL it will soon be able to be renewed as a National lifetime licence and will cost you no more to maintain than a PPL. The IMC privileges are built into a UK National professional licence i.e. you are not subject to the limitations that necessitate an IMC rating and consequently you don't need one. The ANO Schedule 7 lists the privileges of the various licences. If you have a JAA professional licence it does not contain the built in IMC privileges, and if you get rid of your UK ATPL you will loose those privileges and have to maintain an IMC rating at extra cost. I have deliberately not gone into the further complications caused by EASA. If you hold a Class 1 medical when they start dishing out lifetime UKATPLs, get one and hang onto it.