A UK licence as issued by the UK CAA can be changed to a JAR lic as issued by the UK CAA. UK is a member of JAA. However, it must comply with all the JAR requirements.
Note, apart from a certain PPL, a UK ATPL issued on the strength of an ICAO ATPL with only 2 exams sat and a check ride cannot be converted to a JAR ATPL (at this time) unless all 14 JAR exams are passed.
(ie, Australian ATPL, +1500 hours in command (LHS) in 737's, +3000 hours TT can be granted a UK ATPL after 2 exams. Air law and HPL. This I believe, cannot be changed to a JAR one) I am afraid I can't find the reference at the moment. I am having trouble with the CAA's LASOR pdf files.