They will only delete the account if it is inactive for 30 days - quite clear here
http://bt.custhelp.com/app/answers/...happens to email when I stop having broadband
What I have done for legacy email accounts is to set up a Gmail account which accesses the old email account via the POP3 settings. I set it so that Gmail deletes the email from the old server when it imports an email. This stops the old email account filling up with a billion emails. This is just like setting up the account to use Outlook or Livemail etc.
I have legacy accounts from

.net and Tiscali that are still going despite not using their ISP for over a decade.