If you have the ATP you are excempt from a formal course, ie. you can just study for the exams via distance learning, then come over to the UK and sit the exams. Once you have those, to convert to JAA you'll need to do roughly 15 hrs instrument course for the IR conversion, and probably 5 hrs for the CPL conversion.
Idea of cost: around £5000-£7000 depending on where you do the conversion. This assumes you're at a standard to pass first time of course. Where are you in the USA? I know a place in California who could help with the IR conversion preparation, PM me if you need more info.
Rgds
EA