I suggest you read up on your history also Oddlegs
You are wrong. You are very wrong and when someone is wrong on the internet? Well, you know the drill.
Most of the UK fibre network was installed by US company Nynex. I say most because there were some smaller, local players over the years but it was pretty much all handled by Nynex. They ran some cable TV plus dial up internet services and had a backbone to the US.
In 1997, the UK assets of NYNEX were merged with the Cable & Wireless subsidiary Mercury Communications, cable operators Vidéotron and Bell Cablemedia, then renamed as the new company Cable & Wireless Communications. This company offered the UK its first alternative to BT for telephone services and continued to operate cable TV Services.
C&W changed their business focus worldwide in 2000 and decided to pull out of the UK domestic market ( as well as the US corporate market) and sold their UK assets to NTL. They did little to no infrastructure expansion during their ownership.
NTL ran in parts of the UK with Telewest being their only real competition. Some time around 2005 NTL had been running an ISP for Virgin ( Virgin.net) and bought that company out to begin their own ISP service. They also struck up talks for a merger with Telewest. That merger eventually went through in 2006 (top of my head here) but to prevent triggering a change of ownership clause with the BBC for services the smaller Telewest acquired NTL to become NTL:Telewest.
In 2007 this company, now running Phone, cable TV and Internet purchased Virgin Mobile to become the first quadplay media business in the UK. At the same time NTL acquired a licence to use the Virgin brand name from Richard Branson in exchange for shareholdings. So NTL:Telewest was re-branded as Virginmedia but were exactly the same company as previously but now had a mobile phone network in their asset locker.
NTL and later as virgin invested very heavily in infrastructure over the years regularly increasing their reach and are currently undergoing a £3Bn improvement to reach a further 4-5 million homes in the UK and improve speeds and services. Virgin's current lowest package available is 50mb/s with the highest being 200Mb/s. The 100 Mb/s package has been upgraded to 152Mb/s and unlike non-cable competitors those speeds are pretty accurate most of the time. All of this is with fibre to the cabinet and copper to the premises.
So you see my incorrect friend nobody went bankrupt. Nobody paid for infrastructure that a later company benefited from because it was all bought and paid for. If companies have disappeared it is only through the normal business processes of acquisition and merger and in fact both NTL and Telewest exist today as Virginmedia because all they did was change their name.
I hope that helps you brush up on your 'history'.