People (me included) say that " the network worked fine when TeleWest had it" and it did but maybe it lost so much money it had to be bought?
It kinda did however Telewest frigged the network like crazy to avoid spending significant amounts of money on it. They were just about out of room to frig it anymore when ntl bought them.
If you noted which areas were the most delayed for the upstream upgrades all bar one were ex-Telewest. ntl were rebuilding some of their poorer quality networks during the early / mid 2000s so in many cases they were in a position to deploy upstream uplifts quickly. They were also pushing fibre deeper into the network rather than doing configuration changes and pushing existing network harder, Telewest were doing their utmost to keep their CapEx low.
Telewest, basically, spent most of the 2000s waiting for ntl to come and get them and delayed expensive upgrades accordingly. ntl were astounded at the amount of upgrades ex-Telewest needed for 20Mb, considerably more in the field network work was required than the ex-ntl network despite being less than 2/3rds the size. ntl's network was 8.4 million homes passed, Telewest 4.9.