Legislation and government advice
The full name of the legislation which controls the display of advertisements is the Town and Country Planning (Control of Advertisements) (England) Regulations 2007. The part of this legislation that describes the conditions and limitations applying to estate agents' boards is Schedule 3, Part 1, Class 3A.
The government has also produced a guidance document that provides advice on the advertisement regulations in general. The section covering estate agents' boards is on page 14 of the document.