[leoedin]

The problem with making something like computing (or any non-standard) A level a requirement is that it immediately limits people who couldn't study it. Not all schools offer computing (or other less popular A levels), and by making it a requirement you're penalising those who had to attend them. In practice, that means rural and less-well-funded schools.

There obviously has to be some prerequisites. Perhaps the solution would be a summer school for students without A Level computing. Most universities offer a foundation year for people who want to study engineering or maths related subjects but don't have the A levels required. However, given that you only need to cover under 1 semester of work, you'd be pushed to convince people that because they didn't study a computing A level they have to add an entire year to their degree.

Probably the best option would just be to make it a strongly required subject, and cover the basics in less time. If you don't have computing A level, you need to work harder.