Caveat: I work for a UK FTO.
The original JARs did not contain any method of conversion of licences. This was clearly ridiculous, because experienced pilots having worked in the USA or internationally on an ICAO licence had to do the same 55 (or 50) hour course as people who had never flown under IFRs. So for the benefit of such pilots, experienced in IFR flight the 15-hour abridged course was determined as a minimum conversion.
It was never intended as a conversion for people who had trained for an IR elsewhere but never used the rating, never flown instrument sectors on a regular basis. Thus the course is highly unlikely to be a minimum if you do this. I know people who have ended up spending more in total than they would have done in a whole UK course, including one who while a European national had been instructing stateside, using FAA CPL/IR but still decided to book the entire 50-hour course.
Keep this in mind when making your choices; try to talk to some people who have completed each route.
Good luck, however you manage it!