If they were really into desperate measures, their actions would be targeting China (you know, that country that now produces more CO2 than all the Western countries combined and that is set to keep greatly increasing it during the next decade), not the only set of developed countries that actually did something and greatly decreased their CO2 emissions in the last decades.
The people involved in XR are British citizens. If they were to go to China and start performing acts of civil disobedience, they'd simply be deported (or worse).
Instead, they're trying to effect change in a place where they're afforded civil liberties to do so. I don't claim to speak for that movement, and maybe I'm not even thinking along the same lines as they are, but I would guess their hope is that if they change their home country then that example will help to influence other nations.
Each country should be responsible to make their territory carbon neutral. If they have an high or low population density should be an irrelevant point.
Who knows how many more people would have been born if they did not enforce their one child policy or whatever.
You want to shift the goal post to territory instead of population? Sure, lets do this. Wikipedia already has a nice sortable list ready for us [0], with the UK at 1,564, nearly 50% worse than China at 1,133 t CO2/km²/yr.
So, how do we want to define a next target, that allows us to keep telling ourselves someone else is the actual cause of the problem instead of us? GDP/Co2? Scientific papers / Co2? Top 10 watched movies/Co2? Options are unlimited, so I'm sure we'll find something in the end. But maybe we should actually verify whatever we propose doesn't also make us look like the bad guys, since constantly shifting the goal post kinda makes us look like fools.
[0] https://en.wikipedia.org/wiki/List_of_countries_by_carbon_di...
China increased them by 300% in 30 years and is set to continue increasing then.
All in all, UK seting their emissions at 0% wouldn't do anything for global warming. But China decreasing them by 50% would probably save us all. Yet, these so called environmentalists, keep finding excuses for letting China go on with their business as usual, which makes it really clear they are not interested in the well being off the planet but in social upveal and some kind of revolution in Europe to install their ideology (which has very little of environmentalism) in power.