I don't know that there is a general rule in the UK. I think it would also depend on whether you drive an automatic car or a manual.
I've had 3 knee replacements and each time I have started driving a manual (stick shift) car at some time between 4 and 6 weeks.
With a manual car, both feet need to work accurately and in co-ordination. Your left foot works the clutch and your right foot works the brake and accelerator. When you do an emergency stop, both feet need to work firmly, and together.
Before I started driving again each time, I would sit in the car with the engine turned off and practice moving my feet from one pedal to another. When I was certain I could do that accurately and with enough speed and force, I drove slowly up and down our driveway, before going on the road.
You do need to be off all the heavy duty narcotics before starting to drive again.