BTMail Basic is only available if you had a BTMail email account prior to 2022 and are leaving BT Broadband in which case your BTMail account will be automatically "downgraded" to BTMail Basic.
If you are a BT Premium email customer, if you no longer want a BT Premium email account, when you leave BT Premium email your account will be downgraded to BTMail Basic.
It is not available to new customers to BT Broadband.
If you are moving from BT Broadband to EE Broadband your BTMail account will be retained as a Standard email account and managed through your MyBT with your BTID and Password.
See message one in this link
https://community.bt.com/t5/General-email-queries/What-happens-to-my-BT-Email-if-I-move-from-BT-to-N....
See link
https://www.bt.com/help/email/bt-email-products
