It's a grey area in the way it's phrased, but if the problem lies within VM's network, and they are unable to fix it within 30 days, they may offer you the right to exit your contract without you being penalised. This usually relates to a drop in speed and not total loss of service, although I would argue that total loss of service is a speed reduction.
Info below from the Right to Cancel FAQ's section on the https://www.virginmedia.com/broadband/our-speeds-explained page.
If the actual download speed you received to your Virgin Media Hub during the test period falls below our ‘minimum guaranteed download speed’ for 3 days in a row (continuously or intermittently), we’ll confirm that there’s a speed fault, and if we can’t fix the issue within 30 days of your report, you may be eligible to leave us without paying an early disconnection fees............
You can read more about ending your contract in Section N of the terms and conditions https://www.virginmedia.com/legal If we don’t provide the ‘minimum guaranteed download speed’ as set out above (i.e. we have not remedied the speed problem within 30 days of your notifying us or we cannot address the problem) then we’ll offer you the opportunity to end your contract immediately, without an early disconnection fee. In exceptional circumstances (for example where you cancel technician visits or miss appointments) we may extend the 30 days but we’ll always discuss this with you beforehand.
You should allow VM 30 days for their engineers to test your connection (although I can't see how they can perform any testson your connection if it's an actual area fault)
You cannot cancel by email. The https://www.virginmedia.com/help/leaving details the accepted cancellation methods. Click on the View Contact options link which will open a pop up window, so ensure you have pop ups enabled in your browser. You can click on "Chat to us online", however the chat/messenger options won't work if the office is closed.
You can call VM by dialling 150 from your Virgin Media home phone or 0345 454 111 from any other phone. Select options 1,1,4 and 4 for the cancellations department.
You can send your leaving request by registered (signed for) post to: Virgin Media, Sunderland, SR43 4AA including your name, address and account number.
You can also contact VM via WhatsApp on 0780 3089 684
Or you can contact VM on Social Media by either messaging
Facebook: https://www.facebook.com/virginmedia or Twitter: https://twitter.com/virginmedia