Hi Michael - The lines are owned by BT, but as far as I am aware, O2 had their own kit located at the exchange to provide ADSL2, which is why they could provide faster speeds. Sky are in the process of migrating the customer accounts of O2 at the moment, but their customer service is by reputation nowhere near the levels of O2.

BT do however own the fibre network (that they use for infinity) so other providers (except virgin) will be piggybacking off those cables.