Openreach inherited the original infrastructure yes, it was built when it was owned by the UK government, then passed to BT, then Openreach (a BT subsidiary).

Openreach inherited the original infrastructure yes, it was built when it was owned by the UK government, then passed to BT, then Openreach (a BT subsidiary). Openreach have since been responsible for the upkeep, and are rolling out fibre to the cabinet using largely BT's own money, rather than government funded. As to their efficiency, well I'm not sure who else has as similar setup of firewalls between their retail business (BT selling to normal people and businesses), wholesale business (sells to ISPs and telcos), and network infrastructure (sells to BT wholesale and larger telcos), so it is a hard one to judge, but if they could do it with 5,000 people, I'm not sure why they wouldn't. Over the years BT have got rid of an awful lot of staff, they're definitely more efficient now than they have been previously.