FFF
I cannot give you a definitive answer as I have no experience in this.
The whole thing comes down to insurance. If the CAA is happy then you won't get done for flying IFR. Insurance (or lack of it) is all that matters. I would plug away at the insurer for a ruling.
You aren't talking to night flight by any chance? This is normally IFR in the UK. This was mentioned here before. It is routinely done in "VFR"-placarded aircraft, with full knowledge of the CAA, and I would have hoped that if, given various accidents over the years, insurers were not paying out, it would have been publicised.
As regards "serious IMC", how would you define that?
Everything that's not expressly prohibited is permitted. If IFR is permitted, and there is no qualification on that, that includes IMC.