Unfortunately VM don't have a customer facing email service.
You can call 150 from a Virgin landline, or 0345 454 1111, options 1,1 and 4 (Changes to your package) butbest to call at 8am to avoid call queues
You could also try the text messaging service. Just send a text with a description of regrade 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 could also try the above with the thinking of leaving us (cancellation) option.
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali