Also, if you just want to fly UK registered aircraft without having to do the whole JAA conversion, it is possible if the following applies

The UK CAA will issue a licence restricted to G Registered aircraft if you meet the following criteria:

more than 3000 hours total time on aircraft of more than 30,000kg AUW
more than 1500 hours pilot in command on aircraft of more than 30,000kg AUW
type rated on an aircraft of more than 30,000kg AUW
more than 500 hours total time on the aircraft above
able to complete a flight test on the same aircraft (this can be potentially expensive)