Mine are in private school in Edinburgh therefore school is free to choose its own path.
They broadly follow a Scottish curriculum but when the exams were changed a number of years ago they chose to do GCSEs instead of Nat 5s, then back to highers and AH.
They wanted the rigours of the 2 year GCSE course and felt Nat 5 course didn't give enough. They also felt it important to keep the breadth that highers offers.
They looked at all subjects carefully and the vast majority articulate very well. As pp pointed out, there's very little difference what's being taught at primary and early secondary. They have a lot joining for secondary from Edinburgh state primaries and from overseas.
GCSEs have a lot more exam papers though, 4 for English sat over 4 different days.
@TheGenealogist I don't recognise that triple science qualification. That must be a particular exam board they've chosen that offers that. There's 3 different exam boards.
Our school does separate chem, biol, physics.
I suppose what I'm saying is mixing up different systems is not necessarily the big problem people may perceive it to be. The content taught and exam methods are not really that different. Let's face it maths is maths!
As pp has pointed out the issue may be lack of choice.