The speed limit is not a target.
I know where you are coming from though, as part of training the examiners do want you to be at the speed limit if safe to do so, but this is to demonstrate that the driver is competent at driving at the speed limit and if not they might fail the test. Motorways aren't part of the test so if you can't drive at 60mph then you certainly wouldn't be safe on a motorway.
60 mph on the motorway is acceptable, some types of vehicles have speed restrictions and so we have to use the overtaking lanes to make progress.
As for other classes of roads, the speed limit is a maximum and NOT a target. You drive at a safe speed for the conditions for the particular circumstances.
What happens when you get stuck behind, cyclists, horse and cart, tractors, learners or dustbin wagon and there is a long tail back, moan because they are not travelling at the speed limit?
It is a public road available for all kinds of vehicles and road users, yes it is a bummer but we just have to patient and accept it.
There are far more things to be concerned about such as why so many drivers / riders get killed every year.