In terms of PR, if it was in place, I think that Labour and the Tories would clearly and quickly split into multiple parties. They are forced to operate as wide coalitions within themselves under a FPTP electoral system, though Labour are a wider coalition than the Tories, just like the Democrats a wider coalition than the Republicans in the US.
Under PR, there’d be no need for the likes Rebecca Long-Bailey and Rachel Reeves to be in the same party as each other (they clearly shouldn’t be), or Jeremy Hunt and Jacob Rees-Mogg. Even the Lib Dems have operated as a coalition between the Social Liberals (such as Charles Kennedy - RIP) and Orange Bookers (such as Nick Clegg, who seized control of the party and led it to destruction). So there would be a (IMO) sorely needed political realignment. Yes there could well be a coalition between parties that currently make up different wings of the Labour party for example. But crucially they wouldn't be battling it out for the 'soul' of the same party on a daily basis. And while there were many things that the coalition government did from 2010-2015 that I despised, notably excessive austerity (while I'm no fan of Clegg, Cameron admitted that it would have been even deeper without the Lib Dems), Cameron had more battles with his own back-bench MPs during that period, notably over legalising same sex marriage and Europe, than he did with the Lib Dems in his government.
Also in terms of UKIP winning a lot of seats under PR in 2015, well under FPTP and even from outside the Commons, they and Farage (also through the Brexit Party) exerted a huge amount of influence over the Tories and several Tory governments. And a large number of Tory MPs such as Steve Baker, Suella Braverman, Christopher Chope, Peter Bone, Mark Francois etc. are in reality the same thing as Ukippers. They just were never going to give up / jeopardise their ultra-safe Tory seats where for a long time a donkeys wearing blue rosettes would get elected.