I think it depends on how many hours you have on an FAA ATP. I can't remember the exact number (comparatively high I think) but if you have more than this, then you can be exempt the classroom element of the ground school so the theory side would only cost 13/14 exams x £63. If you're not exempt the classroom study, then add about another £1,700 on for the course plus accommodation and/or travel expenses.
How many types do you have on your FAA ATP as each of these would have to be converted?
A UK JAA ATPL includes an instrument rating. Is this conversion also included in your £12,800? If so, then I'd bite their hands off at that price for a full conversion; CPL/IR/type ratings for £10k (after the groundschool) is very good indeed.
Cheers
Whirls