If the van is not operated as part of a "camera partnership", it doesn't need to be marked at all.
Beds police operate unmarked camera vans/cars:
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Bedfordshire Police Road Safety Enforcement Team -
safety cameras in unmarked police vehicles
At the request of parish councils, the Bedfordshire Police
Road Safety Enforcement Team carries out speed
enforcement checks. Speed checks are also conducted
as a result of members of the public complaining about
speeding motorists.
The Speedmaster camera equipment is specially fitted
to unmarked vehicles to detect speeding motorists who
exceed the legal limit. Offenders will then receive a Fixed
Penalty Notice through the post. For more information
about the Road Safety Enforcement Team click here.
http://www.drivesafely.org/safety_cameras/why_cameras.htm