The school I went to in Scotland only offered computing up to a level known as "Intermediate 2", which was somewhere between a GCSE and an AS level. We did some Java programming, and it was quite an interesting class that provided enough of a basis in basic programming concepts that I was able to continue learning, but that was it.
Higher computing? You had to get a 30 minute bus to a local adult-education type college. Advanced higer computer science? I wouldn't be surprised if fewer than 100 people a year actually sit that exam. I did advanced higher maths, and only 400 people sit the exam a year.
The Scottish curriculum is often quite good (AH maths taught me the first year and a half of my engineering degree's maths modules), but it suffers from a lack of students taking it. It's poorly recognised outside of Scotland (AH's are generally harder than A levels, yet are seen on the same level by most English universities), and the private schools that would give it some clout almost all use the English curriculum.
[Apologies for, yet again, playing the role of the bad tempered Scot.]