BritBox Subscription Billed Through Amazon Cannot Be Cancelled Separately

I have a BritBox subscription that was started as a free month through my Amazon account in June this year. I now wish to cancel BritBox only, while keeping my main Amazon account active, but I have been unable to find a clear way to cancel BritBox through Amazon. As an elderly customer I find the cancellation steps confusing, and I have not been able to reach anyone by phone to help me. I am concerned that the BritBox subscription may continue when I only intended to try it for a short period. I kindly request that my BritBox subscription, which is billed via Amazon, be cancelled as soon as possible, while leaving my Amazon account fully active. I would also appreciate written confirmation that BritBox has been cancelled and that no further payments will be taken for this service.