They have a huge majority of MPs of their own, the likely support of almost every MP from the smaller parties, and the Parliament Acts. If they wanted to force

They have a huge majority of MPs of their own, the likely support of almost every MP from the smaller parties, and the Parliament Acts. If they wanted to force through some form of PR in a few years then the only things that could stop them would be a government-ending rebellion by Labour MPs (which seems unlikely - if they're still in trouble then many of those MPs might have a better chance of keeping their positions under the new system) and time (the Lords could delay the change until after another election if the process wasn't started soon enough). It's not a manifesto policy but then they're probably going to implement a lot of things that aren't manifesto policies between now and then. At least this one would have cross-party support in Parliament and probably broad public support as well.