Mark - I believe that you've already had the answer you needed.
In general terms (ignoring the IMC Rating element), the perceived wisdom appears to be '2 out of 3' reqts needed to be met:
UK Registered Aircraft
UK / JAA Licensed Pilot
UK Airspace
Thus, you're OK flying N Reg in UK Airspace with a UK / JAA PPL, OR flying a G Reg in Continental European Airspace with a UK PPL, etc. What is NOT OK is flying in an N Reg in Continental European Airspace with a UK PPL AIUI.
For the latter example, you'd need an FAA PPL
I believe that the IMC questions have already been answered.
See you around the bazaars!