UK speed limit on a motorway is 70mph (112kmh) and on single carriageway roads where no other speed limit is shown, it's 60mph (96kmh).
Built up areas with street lights close together is likely to be a 30mph (48kmh) area, although this is normally shown on a sign as you come in to it.
Loose conversions:
mph. Kmh
70. 115
60. 100
50. 80
40. 65
30. 50
Watch out for yellow boxes on grey posts, they'll be speed cameras and I'm not sure where they're at in sharing information at the moment!!
In the main, use common sense, stick with the traffic flow speed and slow down if you're in a busy town with lots of people about as jay walking isn't illegal here.