Modern speedos are now considered so accurate that some forces - although not all - will prosecute for even 1mph over in high risk roads, red routes etc.
The old 10%+2 was only ever guidance, and was never universally observed anyway.
I think the chances are unlikely, particularly if they do send it to court as it'll be too late to lay a new charge once the postal summons has been accepted. They either have to lay the correct charge, or in some serious cases they can lay multiple alternate charges and let the court decide which to progress, but speeding isn't one of those scenarios.
The old 10%+2 was only ever guidance, and was never universally observed anyway.
I think the chances are unlikely, particularly if they do send it to court as it'll be too late to lay a new charge once the postal summons has been accepted. They either have to lay the correct charge, or in some serious cases they can lay multiple alternate charges and let the court decide which to progress, but speeding isn't one of those scenarios.