The amendment to the ANO is clealy applicable to helicopters not fixed wing.
The obvious purpose of the original legislation was in recognition that the "vanilla" CAA PPL has no means of navigating unless he can see enough of the surface to follow visual references.
It has nothing to do with ability to fly on instruments otherwise the restriction would apply to non CAA "vanilla" PPLs where flight above a solid undercast is usually permitted as long as the visibility is better than VMC. Such "foreign" pilots receive no more instrument training (in term of flight in IMC) but receive more training in instrument navigation. This has changed. It would be interesting to know how the instrument component of the UK PPL syllabus differs from the French component or the current FAA component for that matter.
Logically the technical answer to the question for the sake of technical debate would seem to be can the pilot navigate by visual reference alone to the surface that he can see.
There is a perverse logic because presumably if you take off in VMC, continue over an undercast, but navigate towards a know peek you might claim you were always in sight of the surface.
It is interesting that the thrust of the amendment so far as helicopters is concerned has far more to do with the pilots ability to fly on instruments without reference to the surface. This seems at odds because I would not have thought it was any more or less difficult for a heli pilot to fly in VMC above or below a cloud layer (navigational issues aside).