Probably, English, because it rarely costs that little in reality. Working for an FTO we see people who have tried this route. I have a friend and former student of mine who, despite being an instructor with CPL/IR in the US, took the full UK IR course instead of the conversion because he did not feel his training was up to standard to get the IR on 15 hours. Probably 50 hours is more than most will need, but 15 hours is a lot less. The conversion is designed for people with some instrument experience, having had a flying career elsewhere, not really for people who have only done the US IR course and nothing else.
There have been complaints in this forum from students at other schools who have come back here and been told by their instructors that the training they have had in the US has been as good as useless, the course has ended up more expensive by this route.
Since in your scheme you are suggesting a conversion for people with no flying in UK airspace and no real time under their belts even the CPL conversion would take some time. At that level of experience I would advise people that they should not count on taking less than 15 hours to be of a standard to pass the CPL skills test.