You must make sure which address to send it to, with the customer service line of the company that actually sends you the bills. Just as an example, you will have different addresses for cancelling depending on whether you got your phone from a Vodafone store or CPW.
If you send your cancellation letter to Banbury and you're billed by CPW, I'm pretty sure they will ignore the letter because it's CPW that needs to be notified, not Vodafone HQ.