First, an answer to the literal question: no there is no law dictating that cancellation must be possible via the same method.
However, I suspect you are really interested in a subtly different question, which is whether you have an alternative. You almost certainly do, which is to write to the company at their registered office (you should be able to find this on their website, but if not you can look them up at Companies House - https://www.gov.uk/get-information-about-a-company) stating that you do not want auto-renewal. Be explicit about anything relevant, such as policy number and dates. You don't say why you feel there is no other way to prevent renewal. Is it because your contract explicitly states that the only way is by phone, or simply that the online site provides no mechanism to di it? If they state it explicitly, they may try to enforce that, but I would be surprised if a court upheld it in the face of written instructions. Ensure you leave plenty time between posting your letter and the renewal date. Use first class post (because this is legally presumed to have arrived unless the other side can provide evidence to the contrary), keep a copy, and write down in a note to yourself when and where you posted it as you do it. Anything you write down (on paper!) in this fashion ("contemporaneous notes") a court will find very persuasive (this is because if you produce your note, it means that either you're a liar and a forger, or what is in the note is true, whereas if you just swear to something there's also the possibility that you are mistaken).
And, of course, it's often not enough to have a legal right as the difficulty enforcing it may make it not worthwhile.
