Using New Outlook on Windows for personal email, calendar, and contact management
Hello,
You are using the New Outlook, right?
If so, when you are in its Settings, I would suggest that you remove the BT account. Then add it again.
When you add, it will ask you for the email address, go ahead and enter it.
At the next step where it asks for password, toggle on the "show more". this will then bring up more options for you to set it up as IMAP. I do believe the "use recommended settings" is already checked, if not, you can turn it on. And you should be able to configure everything else according to what BT wants.