When you say that you cancelled your Xbox auto renew last month, when exactly last month? You might have to wait up to 30 days looking back at a post from 2024 before it's sorted.
If you're lucky, a Moderator might be able to assist if they read your post.
There is an "Xbox Escalation Form" that needs to be sent off by the BT Customer Service rep and then a specific team who deals with this will work to de-link the account for you. This should happen within 7 days of the form being sent by the Customer Service Rep.
You will need to contact BT Billing 0330.1234.150 again and request that they raise the form.