Driving under the speed limit

What’s an acceptable speed to drive under the speed limit in normal safe driving conditions?

For example, the speed limit down the motorway is 70mph, yet some people drive only 50mph, which can be dangerous when others are driving 20mph more

In a 60mph zone, is it okay to go 30mph?

Does it depend on whether there are overtaking lanes?

I was stuck behind a learner driver today who insisted on going 30mph in a 60mph zone. Whilst yes, they are learning, in my opinion it’s actually more dangerous to drive at that speed, and if they’re not comfortable going faster than 30mph then they shouldn’t be taking national speed limit roads. I believe for over 60mph there should be a 15mph ‘safe speed zone’ below the limit, at 50mph it is 10mph, and at anything lower it should be 5mph difference