You can get several ISPs in the same street, fed via the same cable network, with widely differing performance. After the limitations imposed by cable length and type, you run into the contention ratio. If a lot of connections are using the same node at the exchange, its carrying capacity is reached, and everybody gets a slower performance. The exact same thing happens with optical networks. They perform great, then they become popular, then crowded, then they slow down. The capacity of the optic is, for practical purposes, infinite, but when it gets turned back into electronics, the physical bottleneck reappears.
The BT/talktalk comparison is probably very local, and just reflects the equipment provision by the two companies in that building, and might, or might not, be repeated anywhere, everywhere or nowhere else. Toss a coin in the air time. BT might give better service - they don't have anyone else to hide behind.