DfT rules for netting off wrote:
Rule 3: Signing rules
Camera signs will be co-located with speed limit signs where permitted and practicable
o For fixed speed and red-light enforcement, a camera sign must be placed not more than 1 km from the first camera housing in the direction being enforced (including or excluding side roads at the discretion of the partnership). For fixed speed enforcement, co-located camera and speed limit repeater signs, or the sign shown in diagram 880 where appropriate, must be placed to allow the signs and speed camera to be visible to the driver in the same view.
o For mobile enforcement, co-located camera and speed limit repeater signs, or sign 880 where appropriate, must be placed in advance of the point of entry to the site or route (including or excluding side roads at the discretion of the partnership) in the direction being enforced. Camera signs must also be placed thereafter at intervals of around 1km throughout the site or route.
Unfortunately:
DfT rules for netting off wrote:
Compliance with these rules has no bearing on the enforcement of offences detected by the use of safety cameras. Non-compliance with these rules and guidelines by a partnership, or representative of a partnership, does not provide any mitigation of, or defence for, an alleged offence committed under current UK law.
So either way you can't use it as a basis for your case.
There's nothing to stop them placing a camera immedately after the speed limit change signs (the new speed limit will be in effect there).