Speak to Retentions on 150 or 0345 454 1111 - options 1-1-4-4 "Thinking of leaving" - they are open 08:00 until 21:00 BUT calling early between 08.00-10.00 midweek is best, when you will usually get a UK call centre with staff more informed and helpful.
Sorry to see you having problems getting through to cancellations, unfortunately there isn't a direct number. I'm at a loss as to why there are no options when calling 150, however you could try the freephone number 0800 052 0422 from a mobile. That shouldn't be linked to your account and so may by pass the no options issue. Fingers crossed that works and you can select the thinking of leaving options.
You could also 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.
Or you can write to them to as per the terms and conditions. If writing send it by registered post.