Not to my mind. We currently have a FPTP system that handed absolute power to a Tory party now riddled with NF/BNP shit like Lee Anderson, Jonathan Gullis etc. This sort of bigoted racist scum would never get to a position of absolute power under PR. Not a hope in hell. They’d get a few gammons on the back benches, but that is the extent of it and they’d be more than balanced out by a very much increased Green and left vote. Splitting the real fascists and nationalists away from the mainstream Tories would also reduce the Tory vote, so that’s a win too.
I’m pretty sure things would roughly follow national voteshare stats with >60% being basically progressive, <40% right-wing. That is a functional system. It will never be as progressive or radical as some of us would like, but neither would it facilitate the system we have now where a minority Tory elite have gang-raped the country for over 90 of the past 120 years. There are some variables, e.g. those who currently understand they are disenfranchised in safe seats and don’t vote are an unknown number and may or may not balance out evenly. I certainly do not fear democracy, though the current FPTP system terrifies me as it is so clearly leading us to fascism.
If in any doubt about PR ask yourself why the Conservatives, like their Republican equivalents fear real democracy so much? The answer is they understand full well that it will end them. They know they can never obtain power democratically as the majority of people oppose their ideology.
But under PR, there is a distinct chance the electorate will return more "NF/BNP shit". Fringe parties would be given a more obvious and 'less-rigged' pathway to power, and then once in power could enter into a coalition with other like-minded parties (or for that matter traditional parties who need the votes to form a government). There's also some research that says that the UK electorate 2:1 leans against left-wing cultural positions (FT), which suggests that even under PR, the cultural right will maintain a powerful voice if not an overall majority. And no turkey will consciously vote for Christmas, so in rejecting PR the Tories are on firm ground there.