There seems to be no
specific legislation which prohibits someone from parking across a driveway (irrespective of whether there's a car in the drive). Here's a list of all the codes, used on parking tickets, by parking officials:
http://www.parking-appeals.gov.uk/RegAndLeg/PC NconCodes.asp
That list shows that (although you can be penalised for parking across a dropped
footway, in London boroughs only), there's no reference to driveways.
However, many motoring matters don't have
specific legislation relating to them. For example, there's no
specific legislation which says that you can't drive on the right-hand-side of the road. That doesn't mean you can do so! (The police have to choose an appropriate piece of
general legislation, such as that relating to 'due care and attention' or 'dangerous driving').
I'm prepared to bet that, if you repeatedly park your car across the Chief Constable's driveway (irrespective of whether he's trying to get in or out of his drive when he finds out), you'll find yourself charged with something like 'obstruction' ;-)
Chris