Banner drops are a common form of protest, and legal consequences are very unlikely.
However, you should be aware that you could be committing a criminal offence, particularly if hanging a banner above a road in an unsafe way.
You should also do some research about the intended location for placing the banner to check whether there are any local byelaws that apply. For example, dropping a banner from a bridge over the river Thames in London would contravene byelaws that apply to the river. There may be similar byelaws in place for particular bridges, so it is best to check.
There are 2 main laws that could apply:
Causing Danger to Road Users
This is defined in Section 22A Road Traffic Act 1988 - Causing Danger to Road Users.
The relevant part of this piece of legislation is:
(1) A person is guilty of an offence if he intentionally and without lawful authority or reasonable cause—
(a)causes anything to be on or over a road, in such circumstances that it would be obvious to a reasonable person that to do so would be dangerous.
It goes on to define dangerous: “dangerous” refers to danger either of injury to any person while on or near a road, or of serious damage to property on or near a road; and in determining for the purposes of that subsection what would be obvious to a reasonable person in a particular case, regard shall be had not only to the circumstances of which he could be expected to be aware but also to any circumstances shown to have been within the knowledge of the accused.
So, the key here is to make 110% sure that there is no chance of the action being ‘dangerous’. It seems pretty clear that if the banner drop is done safely, as illustrated in the previous sections, there would be no offence committed under this section of the Act. This is an either way offence, so will depend massively on where it is tried (magistrates or crown court). There is a chance of prison time for this (7yrs if tried in crown court).
Criminal Damage is the “deliberate or reckless damage” of property without lawful excuse. The damage does not have to be permanent – people have been accused of this offence after using chalk on paving stones. It includes interfering with property in a manner that causes loss, which could include loss of profit (e.g. by setting off a fire alarm). Defence can often hinge on the ‘lawful excuse’ aspect of this offence. This offence is divided into two: Damage below or above £5,000.
If value of damage is under £5,000 Sentencing starting point: conditional discharge and compensation order (i.e. you may be asked to pay back the costs of cleaning and repair), tried in a Magistrates Court. Maximum sentence: custodial sentence of up to 3 months, £2,500 fine
If value of damage is over £5,000 Sentencing starting point: a suspended sentence and compensation order, tried in either a Magistrates Court or Crown Court. Maximum sentence when tried in Magistrates Court: £5,000 fine and six month custodial sentence. Maximum sentence when tried in Crown Court: custodial sentence of up to 10 years.
A suspended prison sentence is the term given to a prison sentence imposed by the court, and then suspended (i.e. ‘delayed’). The court may decide to delay the prison sentence to allow the defendant a period of probation, or to undertake treatment for an addiction, or to meet conditions in the community. If the defendant breaches the terms of the suspended sentence, or commits another offence, they are likely to be sent to prison to serve the original prison term imposed.
You should also be aware of the possibility of 'Intent to cause Criminal Damage'. People found on their way to an action with bolt-croppers have been charged with having items with intent to cause Criminal Damage. The most ridiculous arrests we’ve seen for this were for having permanent markers!
The other potential offence is from the old piece of common law legislation - Public Nuisance.
In simple terms it is an unlawful act which interferes with the lives, comfort, property or common rights of the general public. In practice, it appears to be the fallback legislation that the police use if they can’t think of anything else to use to deal with issues!
All those involved with the banner drop can seek further information and advice through Green and Black Cross: https://greenandblackcross.org/