Hmm.. take a peek at this link:
http://www.speedlimit.org.uk/speed_markings.html
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.