The fact is that TalkTalk have their own equipment based in exchanges, along with other ISP's. This what gives them the cheap cheap price as they don't use BT's infrastructure at an access layer level. (The switching equipment, not the lovely twisted copper pair underground)
Some companies invest more in their infrastructure than others, some may opt for high-end equipment whilst others go for cheap-and-easy equipment.
Overselling can become a factor if an ISP doesn't put in considerable growth measures with expansion, as well as both system and physical measures to control and divide bandwidth accordingly.
TalkTalk does have a reputation for overselling, as well as taking time to replace faulty equipment. Overselling bandwidth is always a big no-no as that's what causes packet loss and disconnections. Luckily if your in an area where there is low-demand or sufficient space and bandwidth, you'll be fine.
A brilliant site to use for information is here
http://www.samknows.com/broadband/exchange_searchIf your ISP has a tick under LLU operator presence, it means they are likely to have their own equipment in the exchange, if not, they are more than likely to be using BT's wholesale network. So in reality your using BT's service from the ground up even if your with a different provider.