Cyclists Dismount

An information sign (white lettering on blue rectangle), rather than a 'no cycling' sign, meaning it does not require you to dismount unless it is already illegal to cycle (for instance on entering a pedestrian area or to use a pelican or zebra crossing).

Cyclists Dismount Sign

Diagram 966, TSRGD.

According to the Traffic Signs Manual it should only be used 

where cyclists are required to use a pedestrian crossing facility that they cannot legally cycle on, at the entrance to a pedestrian area, at a location with a low headroom or width restriction (e.g. a subway or bridge) or at places where visibility is restricted to such an extent that cycling would be unsafe

It should largely have been superseded by the 'cyclists rejoin carriageway' sign, according to the Traffic Signs Amendment (2) of 2011. However it is still widely and wrongly used by local authorities.