Under the Town and Country Planning Act 1990, flags come within the definition of ''advertisement".
Their display is controlled by the Town and Country Planning (Control of Advertisements) Regulations 1992.
• However, under Schedule 2 of the Regulations, the national flag of any country, and this includes the Union flag, is exempt from advertisement control provided each flag is flown from a single vertical flagpole and neither the flag nor the flagpole display any advertisement additional to the design of the flag.
http://www.flagpolecompany.co.uk/index.html