Try phoning in again using the 'Thinking of leaving us' option. It used to be the case that phoning first thing in the morning increased the chances of getting a UK agent but recent topics suggest this may not be so now or ...
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/leaving/
You can send your leaving request by post to:
Virgin Media, Sunderland, SR43 4AA
Use a first-class, tracked 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. I have found that RM signed-for services are a dead loss these days and rarely get a signature so use a service that tracks and confirms delivery.
You can also share your unhappy cancellation experience with OFCOM.
They launched an investigation into VM on 13 July 2023 specifically into VM's cancellation and complaints processes.
https://www.ofcom.org.uk/about-ofcom/bulletins/enforcement-bulletin/open-cases/cw_01275
Case ref. no. and an email at the bottom of the above page or submit with same info via the contact form
https://ofcomforms.secure.force.com/formentry/SitesFormCCTMonitoring
Submitting information won't help your individual complaint but it will contribute to the OFCOM investigation and it sounds like your experience is directly relevant to the terms of reference of the investigation.