I am a Canadian CPL/ME/IR holder doing exactly this. I've nearly completed the conversion, just finishing off the flight training. Here are my experiences to date - hopefully will help:
ATPL groundschool:
Did mine with Bristol Ground School, distance learning, with the two required residential brush ups at Cheddar. BGS are a top operation, very experienced in what they do and get you through with respectable results. The course could be done realistically in about 7 months from home if you put a lot of work in. Nine months would be a bit more relaxed. The INRAT and CPL exams in Canada will barely scratch the surface of the JAA ATPL's, so their benefits are very limited. JAA ATPL material is over worked and over rated, but welcome to JAA land. Distance learning is likely the most cost effective.
Flight training conversion:
Currently converting on the south coast. Regarding the JAA "minimum of 15 hours to convert IR" be aware it is just that. Very few people complete in that time, as the typical ten in the sim and five in the plane will barely give you a look-in at the test routes and exercises required. The biggest hurdle with the IR test is lack of familiarity and complexity of UK airspace, combined with extremely busy RT and unless you are a very well honed single pilot IFR guy, you're going to take a while to get back up to speed in that capacity. Total of 25-30 hours is not unusual. Budget £10K minimum for the IR/CPL. I've yet to do the CPL, but its training as required.