The specific offence Entwood refers to is quite clear;
http://www.legislation.gov.uk/ukpga/1988/52/section/3/enacted3
Careless, and inconsiderate, driving
If a person drives a motor vehicle on a road without due care and attention, or without reasonable consideration for other persons using the road, he is guilty of an offence.
The 1991 amendment to this added the following clarifications - but the core value of the offence remains;
2 Careless, and inconsiderate, driving..
For section 3 of the M2Road Traffic Act 1988 there shall be substituted—
“3 Careless, and inconsiderate, driving..
If a person drives a
mechanically propelled vehicle
on a road or other public place without due care and attention, or without reasonable consideration for other persons
using the road or place, he is guilty of an offence.”
It would have to be shown that the actions taken by a driver (in this instance the middle lane hog) amounted to the relevant constituent parts of the definition of the offence being satisfied so, if a Traffic Officer alleged that such driving amounted to such actions and a court subsequently agreed, then the driver is held to have committed the offence.
In many cases however it may be difficult to prove the substantive nature of the offence without some strong evidence gathered over road distance covered, by means of video, the observation of overt acts towards other vehicles and so on; in many cases the thought of having to try the matter in court is instrumental in some officers giving the driver of the car under notice a ballicking, rather than a formal caution with a view to prosecution.
The wording of the offence as stated in (either) Act is quite open and, in being so, invites the need for adjudication by a court in the event of formal proceedings being taken should the accused driver not accept the allegations made by the police or any other person.