@FuckRealityBringMeABook I work in the substantiality sector, so I know something about this.
Whilst well meaning, people who advocate for degrowth in terms of the population are really uneducated people living in cuckoo-land, who often advocate for depopulation, partly because they want to justify their own choices to be child-free (I'm childess btw, and an atheist).
The population growth across Western European countries in particular is pretty stagnant or declining. That's the population growth of countries and people who apparently care the most about ecological and human rights issues.
Instead, what we'll be seeing in the next few decades is the massive population increases in countries that do not have as a good track record when it comes to the environment or human rights.
Therefore, degrowth in population in the U.K will almost do absolutely fuck-all to help the environment because our decrease will be dwarfed by the growth of other counties, and if they don't do anything drastic to change the way their industries operate or how their population live (which would likely involve quite draconian, unpleasant measures), then the planet will suffer more.
What we are looking at currently in the U.K in the next few decades is an increasingly religious, Right Wing future because those are the people who are having more children. There's a possibility that other factors will mitigate that trend, but otherwise it's a case of mathematics.