I think saying "it's not their network" is a bit unfair. AFAIK, apart from the fibre, it is all theirs.
BTO commissioned the fibre for a business connection in the Southwark area, before they "lit" Colombo House. The comment we had was "you know the rack at the other end of this is empty, don't you ?" Which was OK as the fibre was commissioned in advance of the go-live date.