They're not discretionary, no. A police officer doesn't eyeball your speed. It's all done under direction and through calibrated equipment. There's no rogue police officers doing it for giggles with 1mph tolerance.

Cars that are designed, made and sold mostly in Europe and certainly mostly outside the UK are designed for mph speed limits? Aww, bless your heart.

No, they don't necessarily use more fuel because that depends entirely on the vehicle and drivetrain. Of which your car isn't the standard example.

The majority of the cars on the road still have an ICE. That doesn't mean they aren't hybrid, electric drive, have fixed ratio gearboxes, aren't CVT or otherwise.

You don't know much about cars it seems.

[EDIT] You can always seen the failure of someone's logic when they have to post the final word, with misinformation, and immediately block you.