All signs erected on the public highway must adhere to the Government's statutory directive called ‘The Traffic Signs Regulations and General Directions 2016' (TSRGD), or be specifically authorised by the Department for Transport (DfT).
TSRGD specifies the size, shape, wording and location of signing. A copy of which can be viewed at:
https://www.gov.uk/government/publications/traffic-signs-regulations-and-general-directions-2016-an-overview