Be careful chaps.
If you convert your UK CPL/IR to a JAA CPL/IR then the conversion is done you now have a JAA licence, which you cannot use in a MPA.
If you retain your UK CPL/IR as well, and meet the UK ATPL requirements by 1 July 2002 then you get a UK ATPL
Holding both licences places you in a predicament. You can exercise the privileges of the UK licence and gain Multi-Pilot time, but dont expect to convert your UK CPL now with the 1500/500 hours to a JAA ATPL because you already have a JAA licence a CPL/IR and you don't meet the JAA conversion criteria (namely JAA ATPL level knowledge.
Stick to the UK licence and don't convert until you have to.