> it asks for a password but if I have one I can't recall it.
You absolutely have a password. Every email account requires one.
If you don't know it, that's a whole other thing.
Depending on your ISP/mail provider (I'm not familiar with the specifics of BT Internet), you *might* find the password saved in your MacOS Passwords wallet (open Passwords.app, authenticate, then search for btinternet) and you can try adding this under Mail.app's Mail -> Accounts menu.
Failing that, BT Internet should have some option to reset the password on their site. This looks like a good place to start: https://www.bt.com/help/email/manage-email-account/i-need-to-change-or-reset-my-bt-email-password