I'm not convinced thats correct as my deleted address does not show this - please can you post a screenshot of this error? There may be a way to get your email back, i dont believe the helpdesk always.
Sorry that your email has been deleted. That error message you are seeing is on the Yahoo login page not the BT login page. The BT login page on bt.com/email if you entered the deleted email with the original correct password - it would have given you the correct error message which would have started with "Your BT Mail address no longer exists"