VM have committed to the regulator that leaving will be as easy as joining. See commitment 5 in the Ofcom Fairness Commitments. I will leave you to judge why VM do not bother to offer a simple online cancellation process, why there's regular reports of long delays, calls being transferred between staff who won't help, routinely poor communication, and botched disconnections.
A couple of things I'd correct:
You can complain to VM even as a former customer - just do it by post to the address on the complaints page. Bear in mind that complaints handling is done to the customary Virgin Media standards of excellence, and you should expect a fob-off letter by way of response. If you aren't happy with that, then you escalate the matter to CISAS, the industry complaints adjudicator, noting that you have to either allow VM eight weeks from your complaint to the company, or before that date you have to reject the "resolution" and demand a deadlock letter.
You can also cancel by post without the misery of calling VM. That is a specific and legally enforceable requirement from Ofcom, and involves writing to the same address as before (see VM's cancellation page). Unfortunately as you've found VM handle cancellations outstandingly badly and with a poor grace, meaning that you're more than likely to end up with a formal complaint and a trip to CISAS.
Ultimately, I blame both VM's management and the regulator Ofcom. VM management run their business on a 1990's business model of cheap offshore and outsourced call centres - that's all they understand, they think that's entirely adequate, and they treat Ofcom's regulations like the Pirate's Code*. Meanwhile Ofcom make little or no effort to check that their many rules are being adhered to, and they do next to nothing to enforce them, despite having ample powers to do so.
* "And thirdly, the code is more what you'd call guidelines than actual rules. Welcome aboard the Black Pearl, Miss Turner". I must say there's a certain resonance between Virgin Media and the Black Pearl, that being a chaotic ghost ship crewed by drooling, mumbling zombies, inflicting harm on all who unwittingly come into contact with them.
