Co-Pilot in Outlook

The issue is most likely the mailbox type, not the Copilot licence. Signing in to Microsoft 365 with a BTInternet address allows Copilot in Word and PowerPoint, but it does not make that IMAP mailbox a supported Outlook Copilot mailbox.
In Outlook for Mac, Copilot features generally require a supported Microsoft-hosted or Microsoft 365 Exchange mailbox. An IMAP account can send and receive mail in Outlook, but Copilot may not be allowed to access its mailbox data. There is no Outlook setting that can change an IMAP mailbox into an Exchange mailbox.
Check whether Copilot works with an Outlook.com or Microsoft 365 Exchange account in the same Outlook profile. If it does, the BTInternet account itself is the limitation. Microsoft Support can confirm the current support status for that provider, but this is unlikely to be a provisioning fault.