Try this copied from BT Mail info.
Good news - you can keep your BT (www.bt.com) email address when you cancel BT broadband. To do so, you'll need to sign up to BT Premium Mail. This has the same functionality as your old BT Mail account, though it will cost you a fee each month to keep it going. Sign up by logging on to BT Mail and selecting the option to move to BT Premium Mail, or by contacting customer service.
Any additional email addresses linked to your account will be transferred across to the new service too, so you won't lose a thing.