they can spot check anywhere if its with handheld devices
"Visibility of Police Officers Who Are Performing Speed Checks
ACPO Traffic Enforcement Guidelines are also not mentioned in the law, but they unambiguously state that traffic officers performing speed checks should do so from a position that's clearly visible to the public - take a look.
"31.2 The operator must be clearly visible to the public and the target vehicle throughout the check."
So, if you see a Traffic Officer hiding behind a bush or working from inside an unmarked transit van, why not stop and ask them if they realise that their conduct is breaching the ACPO guidelines - please don't forget to ask them for their name, number and the contact details of their Chief Constable so that you can write a letter of complaint about the officer's conduct."
or do you mean in a van?