If found guilty, what would be a suitable punishment for these two pricks?
More than they will get.
The incident seems to have awoken (temporarily, at least) a realisation that not enough is done to prevent and deter acts like this. However they will be charged in accordance with the laws in force at the time.
Are there any precedents?
Or to take it to extreme, what could you be charged with for demolishing Tower Bridge or a cathedral etc. ?
Practically to hurt someone like this, I’d have them barred for life from holding any licenses for tree work, being a member of any forestry related organisation, and from owning any chainsaw or similar equipment.