I believe they should be able to allocate a BT Mail address for you over the phone or live chat?
The BT help pages suggest a BT Mail address is optionally allocated at the time of placing the BT Broadband order, but there have been plenty of posts from customers who have ended up in a similar situation where they seem stuck without one. Sometimes the mods have helped to get things resolved.
You can use any other non BT email address with a BT ID, but not everybody wants to set up one with a different provider and who may or not have different rules about scanning messages or advertising via email. If you already have a different one available I think you can use it temporarily to get the BT ID set up, then set up a BT Mail address on-line, then amend your account to use the BT Mail one.
