There have been meetings between the UK (presumably on behalf of JAA) and the FAA, but they came to nothing and are unlikely to be revived. there are currently ongoing discussions between UK and Canada at the moment, but I have no idea of current progress.
You can currently exchange a Canadian ATP for a FAA one and vice versa, fixed wing only at the moment.
If you have the relvant experience, you do not have to take formal training, of course, though you must still do the exams.
Phil