I held a CAA SCPL/IR/Multi QFI, but went off the the USA to fly using just a basic CPL. I'm now looking at what is required to get either an EASA ATPL or an FAA ATP
Well, for an EASA ATPL you will need multi-crew time. 500 hours of it. SKS-600 Airship time does not count under EASA I believe as such.
If you still have a current FAA CPL certificate (assuming you do, even LTA-Airship) and have 2000 hours fixed wing (as you say you do) why not look at the FAA route? You will sadly now come unstuck with the new FAA ruling on ATP courses though.
A call to the CAA would however be my first port of call. Is B H (was head of Airship and Heli flight standards) still there? He was my FOI when I flew Airships. He may be a good first point of contact.
I fear the UK EASA route for you may be very long winded. Good luck.