This is what BT have told me via email:
Another service provider has told us you want to move your broadband over to them on 28-Sep-2020. So we're taking this as notice that you want to stop broadband and any dependent services with us on that date.
If that's not right, or you've changed your mind, please get in touch with us as soon as you can. The sooner you call us the easier it is to change things. If we don't hear from you by 2pm on 24-Sep-2020, your services will automatically be moved on 28-Sep-2020.
We'll move you to a new calling plan
Because you're breaking up your bundle by moving your broadband, we'll put you on a new calling plan called Unlimited Weekend Calls . There's no charge for this plan, but you'll still need to pay monthly line rental. The plan won't come with a minimum term contract.
If you'd like to find out more, call us.
Charges for ending your contract
You won't have to pay any charges for ending your contract.
What to expect on your bill
We won't send you a final bill when you move your broadband because you're keeping some services with us.
We'll bill you as normal until you ask us to cancel them
If you'd like to cancel these services or find out more about them, please call us.