a common reason for an aircraft in the UK remaining on the N register was reduction of cost and avoidance of the maintenance rules
That's very true but I'm sure you don't mean all "maintenance rules" are avoided. What do you suggest is wrong with the FAA regulations? The FAA rules are very strict also but perhaps are more pragmatic. The rules being different from the UK or EASA doesn't mean they are inferior. The pragmatism of the FAA regulations is the attraction and most owners tell me that the ongoing maintenance is only marginally cheaper maybe but probably not in practice. Another major attraction for owning an FAA registered aeroplane is the ability to obtain an FAA IR and continue to maintain it, also in a very pragmatic way.
Aircraft are commonly held outside of the country of registration throughout the world, there are many here in France although owned by french nationals who are resident here. The ones I know are extremely well maintained and the pilots conscientious and responsible. The claim that you can do all this only in the UK is wrong.
Whether an aeroplane is owned by a trust and where it is registered is irrelevant. Aeroplanes that are operated legally and safely are not a problem. If the aircraft is not maintained properly and the pilot operates outside of their competence then that is the issue. I believe it is correct to say that all those involved in the 'Sala' flight are British and French and so no other countries regulations can be blamed for their misdemeanors. I don't understand Mike why you think it should.