Problem you may have is, you'll be getting the JAR ATPL and while the UK ATPL exams will still be valid for upgrade, you'll need to meet the JAR ATPL flight experience requirements. These include 500 hours on multi pilot aircraft JAR/FAR 25 and complete a JAR ATPL skill test on a JAR/FAR 25 aircraft with a JAR TRE.....
Experience you may have from years back etc, but you do need to do the ATPL skill test which could be costly if you're not with an airline....
VJW