I heard this was so impossible, I thought I'd share to give hope, as it appears to be happening.
I joined Virgin Media in December 2016 on a Basic TV+50Mbps+Phone package. I found the contention was horrendous in the evenings - typically <5Mbps at Speedtest, although it often was completely unuseable which would imply slower.
They promised a cabinet upgrade for end of March, but that passed. They then promised again for end June, but I had no reason to trust that, so I (rashly) bought a BT Openreach contract, assuming I had grounds to cancel Virgin once I had an alternate:
I then read a lot about people who had found it impossible without the £240 exit fee.
So, rather than waste an hour on the phone arguing the case, I wrote a two page letter with a history of my complaints and responses. I also had a history of Speedtest emails that I had sent to myself each time, and included those. I posted it off, and resigned myself to getting a no.
So - imagine my surprise when I got a call last week. The agent said she didn't even know I'd sent a letter, but had just been advised I wanted to leave, and she was going to process it. I would have to let the 30day notice period play out, but there was no early termination. I would also have to return the boxes.
I've not had a shipping box or termination letter yet, but here's hoping it's on the way. Not quite sure how I got away with it, and we'll see if it eventuates, but I was pleasantly surprised.
To be fair, Virgin's customer support has been surprisingly reasonable every time. And I will miss TiVO. Shame their network sucks.