I hate to say
"24. We have found that most of our customers choose to renew with us each year. To make renewing easier for you, unless you have told us not to, we will renew your policy automatically using the payment details you have already given. If you do not want to renew your policy, you must call us before the renewal date to let us know. It is not possible to offer automatic renewal with all payment methods, so please check your renewal invite when it arrives for further details."
Is indeed correct. Whether a continuous payment authority has been established or not (if debit card payment was used) or a direct debit was set up does not matter.
What matters is Green flag have established in their T&Cs an automatic renewal process for any method thats likely to facilitate it, namely direct debit or a debit card.
The process of gather an "approval code" for a debit card does not matter. If so, they could use stored debit card data to manually rekey the transaction if need be, as the wording of their terms and conditions would facilitate this.
However in this case....
If you do not want to renew your policy, you must call us before the renewal date to let us know
A company which ONLY allows you to call them to cancel a renewal is making an "unneccesary post sale barrier". Under outcome 6 of "treating customers fairly" (something financial services industry must adhere to under FSA regulations) you would have a cause for complaint as
1. Its unreasonable to force a customer into one method of contact. What if they have a speech disability, what if they only have a mobile and not a land line (forcing a high phone charge).
2. As a consequence of having a speech difficulty it was un reasonable to expect you to communicate with green flag in that channel anyway, and it could be counted as discrimination against people with impaired speech not to make themselves available for something as vital (as it is vital) as cancelling an automatic renewal. Basically in other words you have been discrimated against and because of a disability have not been able to contact them.
Such phrases as "posting an unreasonable post sale barrier" due to required telephone communication and "this is not adhering to the FSA requirement of "treating customers fairly" and discrimination against those "with speech disabilities" and "questionable human rights" in a written complaint will see you a refund of your policy and hopefully some additional compensation.
I'd find out how to contact Green Flags complaints department if I were you.