I have a JAR licence about to expire in a few days time and a CAA licence with a year to run. I spoke with Alaistair Smith in FCL or whatever it's called these days and he said there was no problem with just renewing a national licence and letting the JAR one expire.
EASA may change things a bit but the CAA don't know what will be required at the moment.
A company I once flew for (orange tails) paid for us to get JAR licences and it seems require all pilots to have JAR licences.
The current company (red tails on 747's and 340's) will only pay for the national licence to be renewed.
So it seems there is no hard and fast rule and assuming you are working for an airline, you will not be allowed to wither on the vine! With two years to go to your renewal there will no doubt be more changes in the rules before then.
If your airline is flying non UK reg aircraft ie Thomas Cook with foreign reg 330's this may be significant.
Hope this helps!