I converted an Australian (ICAO) ATPL to a JAR one.
I did the theory full time at ATA Coventry. I think that cost £3600 + 6 months worth of living expenses. Exams are at least £720.
Medical which must be done at Gatwick is £250 or so.
Before spending any serious money I would arrange to have that done first as the JAR (UK) initial medical can catch people out. Even current ATPL holders from ICAO states.
I did the flying with a TRTO/freight company called Streamline (now owned by Emerald Airways – separate AOC’s for the Shorts and the HS-748). I did it on the Shorts 330/360. Cost min £7000. I didn’t have 500 hours on a FAR 25 aircraft so I had to do it that way.
You can do the theory part time to save money, a very close friend did it at Bristol Ground School and said it was the best theory provider he’s ever been to. (737 FO)
There are others around too. Cost is in the region of £2000
There are a few operators of HS-748’s still in the UK. Contact them for the flying aspect. (eg Emerald Airways at Liverpool)
Note, you will have to do your check ride with a CAA examiner on board. Their fee is £578 payable before the check.