Well, I think that's the debate at the moment and I don't know the answer: Are the relevant laws already there? Are new laws needed? Is it an issue with the way the police responds, of fails to respond, to these actions?
It’s in everyone’s best interest to keep restrictions as narrow as possible. The problem isn’t the bridge, but other more acceptable venues that would also be restricted.
Allowing protests only in some alleyway where nobody is present; or if a government can simply and easily apply postponement/cancellation tactics: This would be a problem as well.
It's in everyone's best interest that roads, especially main ones, remain open
Specifically the current 'protests', be it blocking roads or criminal damage, do not serve any constructive purpose at all. They are carried out by the 'usual suspects'. They could organise a massive gathering in Hyde Park (and similar in towns throughout the country), for example, that would get huge media coverage as well without causing problems, ah but that wouldn't be 'direct revolutionary action'...
In the US, we have other disruptive events such as marathons; parades; Presidential convoys. All of which can disrupt main roads and aren’t even protected.
Perhaps I’m not understanding, but is it legal to be climbing and stay high up on the cables on a major roadway?