Public Nuisance has begun to be used by the police against protesters relatively recently. It can be tried in either the Magistrates or the Crown Court (it is known as an “either way” offence).
What we do know is that there is a risk of higher sentences and expensive and lengthy court cases that last weeks.
The prosecution have to prove that you did something that is not warranted by law and by doing this you endangered the life / health / property / comfort of the public or you 'obstructed the public in the exercise or enjoyment of their rights' (i.e going about their lawful activities).
If you want to lower the risk of public nuisance being used against you, then you might want to think about the following options: leave an action if police are present or approaching an area where you are protesting; don’t take a role where you look like you are organising; try to be seen to minimise the impact on the public.
You can always watch from a safe distance until the coast is clear and then rejoin. Although the risk can be minimised, arrest for Public Nuisance on protest actions remains a current possibility and so it’s about making informed choices.
It’s worth noting that 'necessity defences' (i.e 'I had to do it to prevent greater harm by (something like) the climate emergency') were ruled inadmissible in the most recent Public Nuisance protest case, and so perhaps this charge is the least favourite for attempting to force the courts to listen.