Odd. The key fact is that the conspiracy was to supply in Scotland, that is within a different jurisdiction to that of England and Wales.Without the A-G's fiat the trial could only be in Scotland.
There are quite a number of offences that require such consent before proceedings are instituted. Archbold does not give a list but notes when it is required for individual offences instead. It is certainly required for offences under the Terrorism Act 2000 and the Explosive Substances Act 1883.
The A-G can also take over and stop prosecutions, a power generally used when someone has brought a private prosecution of no merit, issuing what is called a 'nolly' by practitioners but is properly a 'nolle prosequi' .