There is no email address to write to for cancelling.
As already advised by Client62 if you simply wish to cancel, with no further negotiation or discussion with VM …
You can cancel your services by post as per info below
https://www.virginmedia.com/help/cancel-virgin-media
Use a first-class, signed-for delivery service. This will give you a date/time of delivery when you gave VM your 30 days’ notice to cancel. Also keep a record of the tracking number.
If you cancel during a minimum term period VM may charge you early disconnection fees.