Unless it's changed recently, the UK CAA will issue a JAA ATPL only for use on G-reg aircraft if you have 1500 hours or more in command of an aircraft 30tonnes or more in primarily international operations or similar. You need to sit 2 exams and a CAA observed check ride. (Need a total time of 3000 hours or more as well)
Look through LASORS for the specifics.
If you then want to fly another reg aircraft, eg EI- or CS- or F- etc, you'll need to complete the remaining 12 exams.