Sorry for not being clear, I wasn't referring to pulling people over in their cars in that post.Firstly, requesting a car to stop is covered under the RTA 1988, and there is no requirement for circs - any constable can request any car to stop, carte blanche.
They can request any driver of a motor vehicle to provide their driving licence, home address and proof of insurance, too. This is all well documented.
If they've felt the vehicle was driving in a manner that may have suggested they were possibly impaired, and then there was a smell of cannabis, they can do a test.