I think you'll find that the contract you agreed to allows VM to change prices as and when they see fit, and the exact words are "- Also, please note that whilst we will have told you the standard monthly price at the time you entered the agreement, you will pay the current price at the end of the minimum period, so the price may have increased during your minimum period. Please check on the Virgin Media website or call us on 150 from a VM landline or 0345 454 1111 from any other phone for the updated pricing."
Regardless of that, you don't like the proposed price and want to cancel, and that's fair enough. Now, don't cancel the direct debit unless you want to be hounded, and have your credit score skidded-up. Your choice, but I assure you that cancelling a DD without having properly cancelled a contract puts you in the wrong, and will cause VM no difficulty, but cause you plenty.
There is a postal option and a valid address if you want to reconsider your objection to cancelling by letter. If doing this make sure you use a service that records delivery and allow for the post time and remember you're only starting 30 days notice when they receive the letter.
The 0345 phone number is not premium rate, and would normally be within inclusive minutes on a mobile contract or landline bundle, if not should be local charge rate. Having said that, customer experience of phoning VM is highly variable - some people report long waits, very poor service, and difficulty cancelling.
