In reply to kevin stephens:
I think this hits the nail on the head here.
Most climbers are very supportive of the environment. Wouldn't your time better be spent protesting something that you can have a meaningful impact on, like something where the decision to go ahead hasn't been made yet?
There's nothing meaningful to be gained from climbing in some trees to protest against HS2. HS2 is going ahead and your actions will only be a blip of a roadblock. They will have negligible impact in the long term, costing the government extra money/time which will push up the price of HS2 leaving less money to spend on other infrastructure projects.
Looking at several other countries on the continent who have high speed trains running at 200mph, it puts our network to shame, running at 125mph (apart from HS1 at 140mph).