Dont know all the answers as a certian degree of it of course is commercially sensitive to the providers.
I expect something like this tho.
Sky probably lease openreach link's to carry traffic from the LLU exchanges to their core network, once it hits their core network which they own and dont lease, it is on their owned infrastructure.
Entanet use BT wholesale (WBC) to reach the interconnects, then transfer traffic to backhaul they lease of another provider (I forgot the name).
BT retail and plusnet use WBMC dedicated which is wholely through BT wholesale until it reaches their handover point (typically in london), but the MSIL's are dedicated not shared with other isp's.
Other BTw isp's like aaisp use WBMC shared, which is also routed through BT wholesale before handed over to aaisp, but the MSIL's are shared with other CPs and managed by BT wholesale.
Talktalk I expect is similar to sky, likewise vodafone.
Talktalk resellers seem to have 2 options open to them. They can either use talktalk backhaul all the way to their handover point which is what I think aaisp does. Or they can hand it over at a node to another backhaul which is what I think uno does who uses daisy backhaul similar to WBC.
Of course I stand to be corrected if I have made a mistake here.
http://www.kitz.co.uk/adsl/wbc_wbmc.htm