I now think the CAA rules are basically the same as the FAA rules for flight in class G, and that's no instrument descent below MEA/MSA in class G unless you are on an approved letdown.
That is true for the US [relying on FAR 91.175(a)]
There is currently no such requirement under the CAA regime. IFR flights outside controlled airspace are simply required to comply with the "Rules of the Air". For IFR flights, these waive the "no flight below MSA" rule when the aircraft is engaged in take-off or landing - with no requirement for the landing to be accomplished using a published procedure.
Should the CAA wish to bother a pilot in the UK for such an action, they would need to think in terms of something more generic - Endangering an Aircraft or its Occupants, for example.
2D