Good old T&CP Act:

"Estate Agents Boards

Estate Agents advertising are governed by the Town and Country Planning (Control of Advertisement) Regulations 2007.

* Rules for displaying estate agent boards
* Reporting unlawfully displayed boards
* Contact us

Rules for displaying estate agent boards

How many Estate Agent boards are permitted to be displayed on a property?
The law states that not more than one advertisement, consisting of a single board or two joined boards is permitted. If more than one board is displayed then the first board to be displayed will be deemed as the lawful advertisement.

What can be displayed on a Estate Agent Board?
No advertisement may be displayed indicating that the land or premises has been sold or let, unless an additional advertisement or a statement that a sale or let has been agreed or is subject to contract.

What size should the advertisement be?
Adverts relating to residential use or development are permitted to be 0.5 square metres, or if two joined up boards then an aggregate of 2.3 square metres is permitted.

How long can the advertisement be displayed
The advertisement must be removed within fourteen days after completion of a sale or the grant of tenancy.

How far can the advertisement be projected from the building?
The maximum projection from a building face is 1 metre

How high should the advertisement be?
No part of the advertisement should be higher above the ground than 4.6 Metres, or 3.6 metres in an area of special control

Can the advertisement be illuminated?
Estate Agent advertisements are not permitted to be illuminated."