The only restriction is that you may not hold more than one JAA licence on the appropriate aircraft category (helicopter or aeroplane); there is no restriction on UK licences. I currently hold 4 separate European licences - a UK lifetime PPL(A), a UK ATPL(A), a UK ATPL(H) and a JAA ATPL(A). Perhaps I'll see if I can get a JAA CPL(H) issued and make it 5.....