Any road with regular street lighting has a default speed limit of 30 mph unless signs indicate otherwise. The entry to a 30 mph zone must be marked by a "30" sign on both sides of the road, but no repeater signs are permitted within the zone (on the grounds that they might be confusing if some roads had them and others didn't). "30" roundels on the road surface are however permitted, with a special dispensation.
In some circumstances, where a side road has a lower limit than the main road, only one speed limit sign for the lower limit is needed, on the left-hand side of the road, and no signs at all are needed for the higher limit if there are repeater signs on the main road indicating the limit within 100 yards of the junction.
Where there is no street lighting, the default limit is 60 mph for single-carriageway and 70 mph for dual carriageway roads. This is referred to as the "National Speed Limit" (NSL), and entry to these zones is marked by the familiar "derestriction" sign with a diagonal black stripe on a white background. Repeater signs are not normally used but are not specifically prohibited.
Any other limit requires roadside repeater signs at regular intervals prescribed by law. The mere fact that a road runs through an urban or rural area makes no difference to the speed limit - it is the presence or absence of street lighting. Thus a lit dual carriageway in a rural environment would have a 30 limit unless signs indicate otherwise, and the speed limit through the snooty Cheshire village of Prestbury, which still refuses to have street lighting, would be 60 in the absence of 30 signs and repeaters.
An increasing number of lit NSL roads (for example the A556 between the M6 Junction 19 and Lostock near Northwich), are now being given "60" or "70" repeaters rather than NSL ones. Presumably the thinking is that this makes the speed limit clearer, as there is evidence that the NSL is not properly understood by drivers, but it must add another layer of confusion as they will wonder what is the difference between a single-carriageway 60 road and an NSL one.
20 mph repeaters are not required within a signed 20 mph zone if the limit is in effect self-enforcing through the use of traffic calming measures. However, if there is little or no traffic calming, 20 mph repeaters are required for the limit to be enforceable.
All roads with a motorway designation, e.g. M6 or A627(M), have a default speed limit of 70 mph unless signs indicate otherwise - for example, the A57(M) Mancunian Way in Manchester has a 50 mph limit as it has some very tight junctions. This 70 limit includes the handful of single-carriageway roads with motorway status such as the A6144(M) off the M60 near Sale.
Speed limit roundels painted on the road surface may be used to supplement a system of speed limit repeaters, but on their own do not constitute a legal system of speed limit marking. For a limit to be enforceable, the proper number of repeaters must also be present.
An exception to this is that the 40 mph limits covering large areas of Dartmoor and the New Forest have a specific derogation from the need to have repeaters because of environmental considerations, and are therefore marked solely by carriageway roundels.
All UK speed limits must be in multiples of 10 mph, as there is no requirement for vehicle speedometers to show increments of less than 10 mph (although in practice most have 5 mph increments). Thus the option does not exist to have 35 mph or 45 mph limits as are commonly found in the USA. It is an irritating feature of many speedometers that the numbers shown are 20, 40, 60, 80 etc. when the two most important speed limits to most drivers are 30 mph and 70 mph