Actually I think the whole UK thing is a mess which works only because some things are not enforced.
Look at single engine helicopters. Almost none of these are certified for IFR, but they can fly at night. How does this work? The reality is that nobody really cares whether you are under VFR or under IFR, at night. Everybody sort of assumes it is VFR.
There is the old nut of privileges of a foreign license which carries a night privilege but which is not allowed to fly under IFR unless an IR it attached, e.g. the FAA one. This one has never been settled, although I believe a number of people have asked the FAA and they were not bothered; this view would have been relevant in an N-reg but who knows about a G-reg flown on an FAA PPL?
It's a mess, but there is no evidence that the CAA has ever bothered to untangle it. They just pretend it doesn't matter.
And I do know the difference between VMC, IMC, VFR, IFR.