They wouldn't necessarily need to get into power; a hung Parliament might be enough (I can see that it won't happen with the Tory party having a large overall majority). Labour is unlikely to win a UK General Election on its own, given the collapse in its Scottish vote, so some form of PR that might elect a 'progressive alliance' of, for example, Labour, the SNP, the Lib Dems and Sinn Fein could be a 'least worst' outcome.
The problem with the scenario, of course, is that two of those parties want their nations out of the UK, so might not want an arrangement that continues UK government. But, again, they might see that as less bad outcome than having the Tories in power indefinitely.
Keir Starmer has made encouraging noises:
https://www.labourcampaignforelectoralreform.org.uk
“We’ve got to address the fact that millions of people vote in safe seats and they feel their voice doesn’t count. That’s got to be addressed by electoral reform. We will never get full participation in our electoral system until we do that at every level. I would consult the Party membership on electoral reform and include it within the constitutional convention that looks at wider democratic renewal--including abolishing the Lords and furthering devolution on the principles of federalism.”
