The DfT has published guidance for traffic authorities on the use of traffic signs and road markings. The traffic signs manual gives guidance on the use of traffic signs and road markings prescribed by the Traffic Signs Regulations.
See the DfT announcement at https://www.gov.uk/government/publications/traffic-signs-manual together with a letter from the Secretary of State, The Rt Hon Chris Grayling MP reminding traffic authorities of their responsibility for ensuring that traffic signs erected on the road network comply with legislation.