It’s anecdotal but I’ve heard a significant amount of Anglos talking about how they wish they could vote for the SNP. When both parties are austerity liberals a lot of the success for the ones out of government is in high turnout. If they don’t motivate or even actively discourage voters (see barring a bunch of leftists from running or banning them from the party) they are liable to lose just on lack of votes. FPTP helps with that, which is why they should support the popular vote, but won’t. “Controlled opposition” seems too tame a label for the current Labour leadership and people just don’t bother voting anymore. Maybe Corbyn makes his own “Workers’ Party” and gets to take the blame for spoiler votes in 2025.
They’re even blaming their loss on ULEZ now, rather than on them shooing away every engaged and hopeful leftist activist.