Numbers of voters is unfortunately irrelevant in our system (especially when the other side gets more also), it's more important which voters you win, where they live and what seats you can get. It's a $hit system, but it's been that way for ages, they know the rules, and could probably have changed them long before any of this crap happened.
2010, 258 seats v 306, when Brown got blamed for a worldwide recession you mean, and people were going after Labour for a war the Tories all voted for, so loads ditched Labour thinking they were clever and went and voted Lib Dem (which backfired)
2015, 232 seats v 330, not really much of a change, but the economic recovery was happening, so not easy to beat Tories then, kind of stuck with the devil you know
2017, 262 seats, looks great on paper, but the Tories still got 317, basically gaining in the wrong places, also pretty much a 2 party vote
2019, 202 seats v 365, yes 365! when the Tories were a mess, you can do anything with 202 seats
What was next under JC,150? The trend wasn't up, that's for sure.
You're trying to convince the wrong person, I would vote labour anyway, as I realise it's the best way to get the Tories out. You need to convince those that ditched Corbyn or the Tories that think he was a moron. You can ignore 5 people leaving to go to tories (no matter how irrational they are) and pick up 5 from further left, as the 5 from the further left do not exist (I wish they did), and either way it's still a net loss. So you're either now -5 or -10.
My points gone sailing over your head Andy. You say Corbyn's unpopular within the party. The evidence says that's not the case.