I don't use the BTMail app so not sure if it will allow POP3. If it does you can not change from IMAP to POP3 on the fly. You will need to delete the email account from the app. When you come to set it up you should use the "manual" option rather than using a "Wizard" to set it up and this, if it possible, will allow you to select POP3.
The settings you need are in this link.
BT Email server settings | BT Help
If the BTMail app won't let you use POP3 you could always use another email app such as BlueMail or AquaMail or one of the many others on offer.
