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The derogatory British-English phrase White-Van Man (plural: White-Van Men) designates:
– a male driver of a typically white delivery or tradesperson’s van, regarded as an aggressive or bad driver;
– hence also: a driver of such a van regarded as a social type, usually characterised as an ordinary working man with forthright views.

This phrase occurs with capital initials, without article, as a proper noun, but also with lowercase initials, with an article, as a common noun.

Incidentally, I have found an isolated early occurrence of this phrase, designating an aggressive or bad driver, used before White-Van Man became a fixed collocation; this early occurrence is from the title given to the following letter, written by one Mrs. Powell, published in the Surrey-Hants Star (Aldershot, Hampshire, England) of Thursday 5^th December 1991 [page 4, columns 1 & 2]:

Where is the white van man?

On Monday, November 18, at approximately 5.20pm, my 18-year-old son stopped at the pedestrian crossing outside the Frimley Police Station on Frimley Green Road, when a white van drove into the rear of his Mini.
Would the driver of this white van please have the decency to come forward with the necessary details so that my son can claim against him for the damage to his car.

It seems that White-Van Man was first used, or popularised, as a fixed collocation in a report issued by the Freight Transport Association in 1997.

The following, for example, is from a quiz published in The Guardian (London and Manchester, England) of Saturday 24^th May 1997:

[The Week: page 2, column 7]:

8) Who was declared dangerous when driving?
a) Van Morrison
b) Vin Rouge
c) White Van Man
d) Jean Claude Van Damme

[The Week: page 2, column 1]:

Quiz answers:
[…]
8. C, White Van Man, the generic road bully identified by a Freight Transport Association report.

The earliest occurrences that I have found of White-Van Man used as a fixed collocation are from the following article, by Gerard Greaves and Jane Warren, published in The Express (London, England) of Monday 19^th May 1997 [page 13, columns 1, 2, 3 & 4]—this text also contains WVM, the abbreviation of White-Van Man:

The curse of White Van Man

To the peaceable armada of road-faring Britons, he is the fifth horseman of the apocalypse. Accelerating through one-way systems and urban ring roads, he terrorises his travellers-in-arms with blaring horn, tuneless radio and screaming invective. He is White Van Man—and his flashing full-beam glare is in a rear-view mirror near you.
[…]
But his name may soon be on bricks. Transport watchdogs have now outed WVM—tattoos, beer belly and all—as a deadly distraction in dire need of re-education.
A new report from the Freight Transport Association says it is high time White Van Men are contraflowed into the slow lane by signing them up to a code of conduct which makes them accountable to their battle-weary adversaries.
“Everyone has come across this phenomenon at some point,” says FTA head of external affairs Geoff Dossetter. “Typically, they drive white Transit vans, go too fast, swear at other road users, give finger signs and behave with unbelievable arrogance.”
[…]
The pothole in the FTA’s plans lies in the very nature of White Van Man, whose maverick behaviour renders him the last person on the highway likely to sign up to a self-policing scheme.
Nor has the flaw escaped the attention of the AA. “These motorists are almost always young males who are working to deadlines,” says a spokesman. “They have every incentive to rush to jobs and that will always lead to accidents, aggressive behaviour and dangerous driving. […]”
And what of White Van Man’s view? One driver laid bare his soul with a confessional which could have come from the weekly meeting of White Van Men Anonymous.
“I am a bullying driver,” confirmed 28-year-old Carl Mountford, complete with tattooed forearm and undulating midriff. “I drive a big vehicle so I can push myself in. They should get out of my way and if they don’t, I flash my lights a lot. If someone is blocking the fast lane of the motorway I go within a couple of feet of their rear bumper and force them to move over.
“It doesn’t matter if they are male or female, old or young—I make a living as a courier and the faster I go the more money I make.”

Photograph illustrating The curse of White Van Man, published in The Express (London, England) of Monday 19^th May 1997: