Not 100% sure about the UK license side of things, but if you hold an ICAO CPL or ATPL and a valid instrument rating in helo’s then it is a fairly simple process to gain your FAA ATP. You just need to have the requisite experience listed in part 61, submit for a license verification (which is valid for 6 months) and then take the ATP written and do a flight test.
Your FAA fixed wing PPL is kind of irrelevant to your situation altogether