Driving on the 'wrong' side of the road is not, per se, an offence. (i.e. there's no law which specifically states that a driver must not do it and a driver can't be charged with 'driving on the wrong side of the road'). If a police officer sees someone doing it, he must decide whether it is likely that a court would rule that the driver is in breach of any of the more general laws which relate to driving a motor vehicle (such as 'driving without due consideration for other road users', 'driving without due care or attention' or 'dangerous driving').
So, for example, the milkman who drives his milk float along the 'wrong' side of a deserted road at 4am is almost certainly acting entirely within the law, even if he carries out the whole of his round in that way. It's up to the police (and later the CPS and courts, if appropriate) to determine whether the actions of other drivers, such as Royal Mail employees, fall foul of the 'general' motoring laws referred to above.
Chris