If you wish to convert your non UK ATPL to a JAR ATPL, you must:
Hold a valid ATPL issued by an ICAO member state.
Have passed the JAR ATPL written exams.
Have more than 1500 hours as pilot of aeroplanes, including amongst other things, 500 hours in multi-pilot operation and 100 hours at night.
Pass the ATPL skill test.
You are exempt any formal training, provided that you hold a current type rating for the type on which you will complete the Skill Test and have logged 500 hours on that type. If you haven't you must complete the appropriate type rating course at an approved TRTO.