When I am driving my car, I use the (very rough calculations) of:
20 & 30mph = double and take off 10
50mph & 60mph = double & take off 20
For the other speed limits other members have said what to use.
Of course this country is only one of a small number in the world which not only insists on using antiquated measurements for most roads & mainline railways, but also uses different measurements depending on its fancy. Why do we measure CO2 emissions in g/km but measure fuel usage in miles per gallon? It's even more confusing as petrol is sold in litres!