Cost is trivial compared to the cost of inaction. The consequences of no action on climate change will have enormous financial cost times ten, times twenty. Do we really want to find out?
You might want to look at Labour, SNP and Lib Dem climate policy. Whether they get to form a government or informal coalition is, of course, another matter as environment is not - yet - the sole issue that wins elections. Even the Conservative party is moving in the right direction, albeit glacially slowly with some creative accounting.
The only choice left is matter of degree.
You might be interested by "The Happiness-Energy Paradox: Energy Use is Unrelated to Subjective Well-Being".
Earth’s per capita energy use continues to grow, despite technological advances and widespread calls for reduction in energy consumption. The negative environmental consequences are well known: resource depletion, pollution, and global warming. However many remain reluctant to cut energy consumption because of the widespread, although, implicit, belief that a nation’s well being depends on its energy consumption. This article systematically examines the evidential support for the relationship between energy use and subjective well-being at the societal level, by integrating data from multiple sources, collected at multiple levels of government, and spanning four decades. This analysis reveals, surprisingly, that the most common measure of subjective well-being, life satisfaction, is unrelated to energy use -- whether measured at the national, state or county level.
(Also hard to imagine that legislators in more climatically-significant countries like China are likely to see climate anarchists shutting down British airports as a sign that more action needs to be taken on emissions and not a sign that certain parts of the West have lost their minds over the subject of climate change.)
The government's net zero strategy is little more than not disputing the Select Committee on Climate Change's report. As the committee themselves note there now needs to be actual policy. From an administration that has been hugely counterproductive to adoption of greener energy.
Edit: I suspect Boris forgot about his pledge the morning after he uttered it. :)
XR are not interested in making friends, but causing enough disruption to get government to treat the issue seriously, in the manner of civil rights and universal suffrage actions. They've certainly increased awareness and discussion. That they actually seem to be making friends by doing so would seem a surprise bonus.
Not only that, the signs are it's working, as change (insignificant tokenism, but nonetheless a step) has already achieved. Between XR and the school strikes parliament has actually declared a climate emergency, and even prospective Tory leadership candidates have declared positions on zero emissions. I don't expect the Tory party to actually do much, but it's a hook on which further legislation will be hung as XR and others are aiming to make it impossible to ignore.