If you have an FAA ATP and a rating on ERJ145 with 500 multi crew hours you should only need to do an LST on the ERJ with a JAA examiner to be credited a JAA multicrew IR and JAA ERJ rating.
Regards to the exams if you sat the old UK CAA exams (as opposed to UK issued JAA Exams) then the fact you haven't completed an UK IR it is likely you will have lost the credits from these. Hence as mentioned above you will need to take all 14 exams.