If the settings are being changed back from the correct BT settings to the incorrect ones this would not be done by BT so it would appear to be Outlook causing the problem and not BT.
Microsoft populates the email settings automatically when you set it up using the "wizard" so it appears that they have the incorrect settings stored for BT.
Make sure that you are changing both the Incoming and Outgoing server settings and not just ticking the box to use the same as incoming.
If that does not help it may be that you will need to delete the account from Outlook and then set it up again using the manual option and populate the settings with the correct ones.
As you are using IMAP all your emails should still be on the BT servers and should just replicate in Outlook as they are showing now.
See link from Microsoft about changing settings.
Change or update email account settings in Outlook for Windows (microsoft.com)