ATPL(H) is probably the most sensible route.
But lets look at things realistically. You will have to be signed on with a course provider of some description or another before you can take the exams.
If you opt for a distance-learning course, which is cheapest way, then how are they going to know if you did 650 hours of study, or 5?
If your theory is up to the standard, then you won't have to do any more training than necessary. My guess is you will find the course notes and associated brush courses essential, far from being the waste of time many believe it would be.
This is in no way aimed at you personally, mortennb, but many people grumble profusely about the licence conversion. Particularly high time ICAO ATPL holders. Well, there is no minimum theory training requirements for them. If they believe their knowledge and understanding is on a par with that of the JAA system then just sit the exams. If it isn't, then the JAA has every right not to afford you a licence.
Any arguments about whether you actually need to know half the subject matter for the JAA exams is irrelevant.