Are you sure that old ADSL backhaul arrangement equally applies to fibre installations ?
TT use their own backhaul for FTTC. They purchase a ports in the Openreach headend and connect them directly to their own infrastructure much as BT Wholesale, Vodafone, et al, do. The only difference between this and the TalkTalk ADSL product is that TT ADSL has backhaul from every exchange, FTTC headends live in fewer exchanges.
Building better networks, not just faster ones.