The CAA has set no time limit, the EU has. The UK cannot issue National licences after June 2002 and so you have about 18 months to get a UK ATPL.
However, do not despair you will not lose your ATPL exam credits just because you don't get a National licence. All you have to do is to continue flying on the CPL/IR you already have. Once you have logged 1500 hours of which 500 hours is multi-crew time, you can get a JAA ATP(A).