I'm afraid VM don't have a customer facing email address.
You could try the text messaging service. Just send a text with a description of cancellation to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible.
You can also call 150 from a Virgin landline/mobile , or 0345 454 1111, but best to call at 8am to avoid call queues, or late afternoon around teatime
Or write to them to as per the terms and conditions. If writing send it by registered post.