I don't have a CAA IMC rating, and probably never will, but from what I read on the Net, the privileges it grants within these areas don't appear to be "true IMC". From what I find, visibility has to be above 3km. That means you have to be clear of clouds, can probably also see the ground, and so you are not totally on instruments. Take off and landing visibility with an IMC below cloud is 1800m. If that is correct you certainly cannot fly in clouds, unlike an FAA IR can in controlled airspace, including that around an airfield field. I can even fly with just a FAA private pilots licence in an airfield environment with less visibility!! (Special VFR, 1.6km visibility, clear of clouds).
If I misread anything you said or misunderstand IMC privileges then I will be delighted to stand corrected.
I'm pleased to be able to delight you then!
An IMC-rated pilot in classes D to G airspace is limited only by the restriction that the visibility for take-off and landing must be at least 1800 m. Flight can certainly be in IMC, and in cloud. Apart from the visibility restriction, it differs from the privileges of the IR only in that IFR in classes A to C are not permitted (as well as a slightly quirky increase in vis for SVFR to 3 km).
The details are in Schedule 8 of the ANO.
Moral: Don't believe everything you read on the Net.