The international standards relating to Transition Altitude/Level leave a lot of scope for interpretation - hence many different arrangements. The rules for altimeter setting in the UK are described in the AIP - like many other things, other States have slightly differing rules. As I recall (without checking the good book) in simple terms, from an ATC perspective, on the way up, when cleared to a FL the pilot can set 1013 and make subsequent level reports as FLs, even if below Transition Altitude. On the way down, when cleared to an altitude, QNH may be set and level reports, even above Transition Level, may be made as altitudes. Some operators have SOPs that mean altimeter setting will be changed at different points in the flight. And terrain clearance is always a concern when down at levels where terrain clearance is not assured.
These generalisations really apply to IFR flights in controlled airspace. Other flights have to interpret the rules in a sensible manner appropriate to the flight profile.
My reading of the rules is that the quadrantal and semi-circular rules only apply to IFR flights.
And in the UK, night flight is not 'considered IFR' but must be conducted in compliance with the IFR. Rather fortunately, although the legislation is a bit convoluted, if the aircraft flies at or below 3,000 feet above mean sea level, at a speed of 140 knots IAS or less and remains clear of cloud, with the surface in sight and in a flight visibility of at least 5 km (yes, a bit like flying visually at night), and follows all the normal practices to do with ATC etc. it is pretty much in compliance with the IFR.