You'll be refunded automatically, don't cancel the direct debit until you receive the final bill and any pro-rata refund.
BT bill in advance, the billing system doesn't factor in any pending ceases, what ever you had when the billing cycle ran is what you're advance charged.
Once your services end, a final bill is produced around 7 days later, this will update to see you were billed for services that were terminated and automatically refund anything owed.
If a direct debit is cancelled, any refund will be sent as a cheque to the registered billing address on the account.
