That has never been true. The police have always been able to operate hidden speed traps, including cameras. Even when the bright yellow ones were compulsory for the safety cameras the police could set a hidden one a couple of hundred yards down the road.
The signs are there to get more drivers to obey the speed limits as are the high vis cameras.
They don't need a new offence, if you take less than a certain time to cross the zone you are speeding, its very simple. They can have multiple entry and exit cameras if they want. It doesn't make much sense to time people across the lights but they can put cameras on lights. If you look at traffic lights some of them now have boxes on sticks. These are aerials in the main but I have seen some with cameras. These are not the same as the sensors for traffic which you also get on some lights where they have stopped slitting the road and putting in wire sensors.