Some notes about Regulation
The British regulatory model is an evolving one. It started out as a reaction against both the nationalised industry controls, and the perceived failures of US rate of return regulation. In the mid 1980s, the zeitgeist was one of markets, liberalisation, competition and private ownership. RPI-X was invented as a deliberately simple and temporary way to mimic the market, but only for so long as it took for competition to do away with the need for regulation. This applied to BT in 1984, as it was expected to wither away after 7 years.
In practice it has gradually morphed into a detailed and permanent set of interventions for the core monopoly infrastructures. Regulation now controls price and outputs, and it has developed techniques to address the asset bases, the cost of capital, the operating efficiency and capital expenditure.
Regulation, built in this piecemeal way, has become heterogeneous. Each regulator has followed a specific path. OFGEM pioneered RIIO, with 8-year periods, whilst OFWAT sticks to 5. OFGEM has indexed the cost of debt, OFWAT has not. ORR has had to accommodate a much greater role for the state, whilst OFCOM has struggled with the emerging USO for broadband. Large infrastructure projects have attracted their own unique contractual frameworks. Hence, the detail matters.