You might be able to regrade by logging into My Virgin Media at the top of the page, although you will probably see an oops something's gone wrong messsage.
You could 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 maybe try the Text Relay Service, freephone number 18001 0800 052 2164 ( Full details of this service and how to access are available here just scroll down to the section heading Relay UK (Text Relay) Service
Is it possible to have a friend/relative with you so you can call, and when you complete account validation, ask for them to be able to speak on your behalf?
If so then you could call 150 from a VM line, or 0345 454 1111, options 1,1 and 4 (Changes to your package) butbest to call at 8am to avoid call queues
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali