They don't have to give you a copy of the evidence until it due to go to court.
That can't happen until they know you will not be paying the fixed penalty.

The easiest way to get out of it is to let it go to court and hope they can't find or don't bring the evidence, happens quite a bit apparently. If they do present it the penalty is higher if they succeed.
The Police don't get the income from cameras to spend on what they like, it's only used for more cameras and other road safety issues.
There only has to be 2 Speed limit signs, to show actual change of limit, either side of the road at the start of the limit.
All the other signs you see giving advance notice of speed change are to encourage people to slow down, not legally required.
I'd guess the End of Motorway sign is a legal requirement though and could be a warning of speed change.

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