Separated or insulated?
In their current form they are bad enough because they are not allowed to deal with consumers directly.
Instead, if you want to complain because (in the hypothetical situation of course) they are so incompetent that they would keep sending engineers to the wrong (new build) estate for a whole year, leaving the occupants without even a phone line; the only course of action is to contact those at director level of BT, who of course are on first name terms with those of OpenReach.
If they are fully split, surely this would just be even worse? Or would they be forced to be accountable to the general public?