[holding answer 3 November 2008]: The Department for Transport has not authorised any signs aimed at general road users that display metric measurements only.
For a traffic sign to be lawful, it must be either prescribed in the Traffic Signs Regulations and General Directions (TSRGD) (2002) or be authorised by the Secretary of State. In Scotland, Wales and Northern Ireland, the devolved Administrations authorise non-prescribed signs for the roads in their areas.
TSRGD provides for the use of imperial measurements for distance and speed. Certain prescribed traffic signs may show metric units of measurement together with the corresponding imperial units—such as signs indicating maximum height or width restrictions. However, metric is not permitted as a replacement for imperial on such signs.
The Department has authorised the use of driver location signs and a limited number of distance marker posts for the trunk road and motorway network. These indicate an established referencing system, which uses metric units, to enable the emergency services to locate the scene of an incident without any confusion.