To be fair, A work colleague and I both recently closed lovefilm accounts and we didn't get the lost disk problem, but I did get a cert of posting before sending not that thye would have known. Saying that I have had problems with freezing the account before and ended up paying for an extra month before cancelling.
It seems that the best way to cancel could be to first report the disks missing and then put the cancellation in
Seeing as they have taken another months money, can you not reopen the account to get the months credit, mark the film as lost and have a few more rentals before closing again.