You have three choices:
1. Achieve the experience requirements for a UK ATPL before 01/08/02
2. Maintain your UK CPL/IR until you reach the experience requirements for a JAA ATPL. This includes 500 hours multi-crew time but there is no time limit and you retain your ATPL exam credits.
3. Convert your UK CPL/IR to a JAA CPL/IR, in which case you lose your ATPL exam credits. Obviously you would only do this if you were after a job that demanded a JAA licence since it would mean having to pass the JAA ground exams before you could get a JAA ATPL.