"Extended post-flight driving should be illegal. It isn't, but will only become so once fatalities occur."
It could very well be subject to the requirements of the Health & Safety at Work Act 1974 (Criminal Statute Law)and its myriad of attached Regulations, if the driver is commuting to or from a place which is not his normal place of work. The employer then has a duty to ensure that the driver and others are not put at risk by work related driving actvities. That good old buzz phrase 'risk assessment' comes into play, as does consultation with employees.
The following document gives a bit of a pointer.
http://www.hse.gov.uk/pubns/indg382.pdf
Have a read of Page 5.