Totally new type rating I'm afraid, if you had 500hrs on type then you could transfer it across with an LPC conducted by a UK TRE on type. Either cough up for a new JAA type rating or do another 200 hours on your FAA ATP.
Once you have the 500 hours then its fairly straightforward, you just fill in the new type form present a copy of your JAA LPC form and they will type you.
You seem to have fallen between two stools at the moment.
PS if you do a type rating ensure that the LST is done by a UK or JAA TRE and not one of the Fsi guys whose JAA license numbers start with 100 as this will also be valid for ATPL issue