You mean like how OpenReach is in charge of all the standalone phone lines (Except for specific areas like Hull)?
That works well as long as you have good government oversight and strict regulation, otherwise you end up in the situation we have currently where:
1. ISPs can blame OpenReach for every problem, but the public can't speak to OpenReach directly so there's no way of knowing what's true or not
b. OpenReach engineers are thin on the ground so problems take ages to get investigated and usually involve a fee for the customer if no fault is found, which it won't be if the ISP is at fault
†. OpenReach or their awful subcontractors like Kelly Communications make very poor targets-driven decisions like stealing another customer's phone line to get a customer online on target because they then have a new target to fix the "borrowed" customer
iv. Despite the government regulation there's still a pervasive sense that BT Consumer has better access to OpenReach than the other ISPs who have to use OpenReach services
Oh, and since BT owns EE and gets OpenReach to run it, the most likely outcome of this nationalisation would be that OpenReach takes over everything and we therefore get whatever quality of network EE currently provides, but everywhere.