I don't think private flight in a single engine helicopter under IFR is forbidden in the UK. All you have to do is observe the requirement of Rule 6 of the Rules of the Air Regulations 2007, and have appropriate license privileges and an appropriately equipped machine.
I can't at the moment find anything prohibiting private flight in a single engine helicopter in IMC either at the moment, although you'd have to be careful of Rule 5(3)(a) of the same regulations. That's the part of the low flying rule (Rule 5) which talks about the failure of a power unit not endangering persons or property on the surface.