I did this about 4 years ago.
I qualified for the bridging package for QSP(H) to frozen ATPL(H), which only required Air Law, Ops Procedures and a bridging tech exam. (It should also have included a Performance exam, but the CAA hadn't created it yet.) Then just the paperwork and the inevitable cheques. I used Bristol Groundschool, who were good at knowing which hoops I had to jump through.
I then bridged from frozen ATPL(H) to frozen ATPL(A). I did the exams through Bristol again, which were PofF, Ops Procedures, another bridging tech/systems exam, and Performance. I did it all by distance learning but given my time again, I would want some face-to-face time, especially for performance.
I already had some 400 hrs FW time, so I didn't have to do hours building, but I did have to restart my PPL(A) and do a qualifying cross-country. I then did a modular CPL course, which was reduced to 15hours (I think), a twin rating (6 hours) and a modular IR Course reduced to 25 hours. Its all in LASORS.
That, plus more paperwork and cheques, got me a frozen ATPL(A). I was exempt MCC due to previous helicopter 2 pilot time, but I chose to do a Jet Orientation Course on a B737 FNPT at Oxford. I would recommend it -it made a big difference for sim assessments at interview time, and also gave me a start when I started the type rating.
If you want to know more, PM me. However, it may not be the best time to be looking for a job as an airline pilot.
All the best.
TH