The problem with "BR plc" was that it was inherently monopolistic and closed to new entrants.
Maybe the privatisation went too far in breaking up the railway, but the regulated TOC policy did allow new entrants, notably in freight and open access.
BR (and the unions) had spent years trying to keep new entrants off the system.
With multiple independent TOCs, you then need a regulator and open system rules about driver/train acceptance and safety cases which we now have.
Ironically, if we had not privatised early, and stayed in the EU, we would now be breaking up "BR plc" in the same way the EU has broken the state monopolies across the continent.