I see your point, thanks, but plusnet had 6 days since the transfer date for issuing the cancellation bill before charging my account. I hoped the bill would be cancelled before taking the money but that's not been the case.
I can imagine that if I had had problems with the new provider the service would have stopped for as long as those problems had taken to resolve with the new provider. Plusnet just stopped the service on the transfer date and that was it for them.
Additionally, I got the "your bill is ready to view" email 2 weeks after receiving the "Leaving Plusnet" email, and the latter stated that "You'll receive a cancellation bill with the charges you need to pay.", it did not say that I will get billed for an additional month and then billed again a final bill.