no
In the UK the speed limit comes into force as soon as you pass the speed limit sign. As such the distance is zero as you should have slowed down to the posted speed limit before passing the sign.
As of today (June 11, 2013), the speed limit was raised to 65 for cars, 45 for trucks. Previously, the limit was 55/35.
If they are doing the speed limit, there should be no reason to pass them. If you are exceeding the speed limit to pass someone, you could get a ticket particularly if they are exceeding the speed limit.
The posted speed limit.
The speed limit is exactly that - the speed limit. While a reasonable excess of speed might be overlooked when overtaking another vehicle, there is no guarantee. You can be pulled over and cited for speeding, still.
The average speed of any car should be the posted speed limit.
Knowing the speed limit
Knowing the speed limit
As close to the posted speed limit as possible.
true
Generally 5 mph but you should always check the particular parking area for the posted speed limit.