EnglishAl: Sorry if this has been said earlier, couldn't spot it on first pass:
Your original statement is not quite true.
>>"What this means is that an FAA PPL ME IR + medical holder is entitled to fly a G-Reg ME aircraft in IMC in the UK on their FAA certificate with NO conversion. "
FAA licence, FAA/IR, g-reg aircraft, UK airspace combinations means you can fly IFR (and also in IMC) in Class F and G airspace only. You couldn't do approaches into any Class D or E airfields in IMC, which of course is where most of the instrument approaches are! So to that original statement you have to add 'providing you stay in Class F or G airspace'