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section 19 of the road traffic act, apparently:
19 Prohibition of parking of HGVs on verges, central reservations and footways.
(1)Subject to subsection (2) below, a person who parks a heavy commercial vehicle
(as defined in section 20 of this Act) wholly or partly—
(a)on the verge of a road, or
(b)on any land situated between two carriageways and which is not a footway, or
(c)on a footway,
is guilty of an offence.
You can't park commercial vehicles of 7.5 tonnes and over on the street overnight. Below that (transits etc), it's all fair game.