On the subject of Greenpeace and air pollution, there was a well-publicised stunt by Greenpeace earlier this year (in April) in which they placed anti-pollution face masks on a dozen or more statues in London, including Nelson's column. As I remember it, the idea behind the stunt was to encourage voting for Remain in the forthcoming EU Referendum through reminding the public of the key role played by the EU in imposing environmental regulations on the UK.
Much less publicised were the subsequent small fines imposed on the Greenpeace protestors. James Delingpole, writing in the Daily Express, gives some details:
Apparently the Nelson's column protestors caused £35K of damage to the statue, but were only fined a total of £115 by a sympathetic judge. Delingpole expands on the general idea of 'judicial activism' in the article.
To give some perspective on this issue, the general practice in the UK nowadays seems to be that people who deface public property are given quite a hard time. Graffiti artists, if caught, can serve prison terms as described in this article:
So if you're deface public property for a cause approved by the UK's liberal elite, you can get away with a mild ticking-off. If you deface public property for your own entertainment, like the graffiti artists, you spend time in prison.