Question Topic: Road and traffic signs
The official DVSA guide to driving states:
20 mph zones
Some traffic-calmed areas are indicated only by a 20 mph (32 km/h) speed-limit sign.
This speed-limit sign, in addition to advising the maximum speed, indicates that there may be traffic-calming features within the signed zone; these may not be individually signed.
You should drive at a steady speed within the speed limit, and avoid frequent acceleration and deceleration within these areas.