You didn't look very hard, Nigel.

Labour leadership: Huge increase in party's electorate

From the BBC article - Labour received more than 160,000 applications to vote in its leadership contest in the final 24 hours of registrations, the party has said.

The number of applications via trade unions more than doubled and the number of party members and £3 registered supporters also rose.

It takes the potential total electorate in the leadership contest to 610,753.

Took me aabout 15 seconds to find that on Google.

I fully accept that 600000 people did not join the party then but there were over 600000 members. There was a massive surge and the membership has now declined under Starmer.

As ever, if all you can do is nit pick at flawed wording, your argument is lost.