Sorry I can’t offer a solution but I just wanted to say that I have had exactly the same experience, including the attempt to make me upgrade even though my broadband worked perfectly until about a year ago. I attempted to change provider only to discover that BT will continue to charge me £7.50 monthly to keep the email address I have used for over 20 years. They are despicable 😡
They will only charge you £7.50/m for Premium mail, if you downgrade to Basic mail you can keep your email address free.
Basic mail cannot be accessed via mail apps or clients, just webmail.