There is no such legal requirement, they can hide them as much as they want.

The reason people think that there is a requirement is because of the rules around "Safety Camera Partnerships". If an area chooses to operate a camera partnership (where the local police get a portion of the speeding fines) then the cameras have to be clearly visible. Even this isn't a legal requirement as such, it is just part of the rules of signing up to the scheme.

Not all areas signed up to the scheme, in those areas all the fine goes to the treasury and they have no compulsion to make the cameras visible.

That said, it is fairly rare now to see one hidden behind trees or signs.

So who here in the UK has picked up the most speeding points over time ? I've only had 3, I managed to get them a couple of months after buying my Subaru Impreza, the temptation to cruise at just under 100 on the motorway was just to much for me for a while...