go on chat and ask to talk to someone from customer relations and they can deal with this or the first person may ask you why you are wanting to leave but they tell you it could take up to 24 hours to talk to someone due to covid19 but thats the only other way.
Virgin added the text messaging option at the start of Covid to help people cancel cancel instead of being stuck on the phone, however replies aren't instant. 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. It may take a few days.
You can also call 150 from your Virgin landline, or 0345 454 1111, but best to call at 8am to avoid call queues
Or write to them to as per the terms and conditions. If writing send it by registered post.