Would a regulated asset model be more economically efficient than the PFI concession model? 

A Regulated Asset Base (RAB) model clearly implies a greater structural change than simply on equity returns. In the regulated utility model, where customers pay for the provision and maintenance of the assets by a naturally monopolistic operator, the RAB model is a way of passing the cost of finance risk directly to customers in line with the regulatory cycle, in return for the low cost of finance delivered by the implicit guarantee of financeability. 

The operator holds the assets in a broad and relatively diverse portfolio as part of a business model in perpetuity. The operator requires itself to maintain the assets in serviceable condition in order to deliver services over them and to meet legal / regulatory requirements. Given the diversity of the asset pool, and the charging regimes in place the failure or unavailability of any particular element of the asset base is likely to only have a very limited impact on profitability let alone debt service. Furthermore, at any point in time, the assets under construction are a very low proportion of the total asset base, with the risks associated with construction outturn borne by equity within their overall returns in so far as they are not transferred to contractors. The economic regulator takes a view on efficiency improvements in maintenance and asset creation but is generally hands-off in terms of specific assets and the renewal / expansion programme.

Most of these characteristics are quite different from a programme of investment in social infrastructure, or even roads (setting aside tolled roads) where the Government itself (or public sector bodies) pays for the assets and has a direct role in setting requirements to quite a detailed level and prioritising investment. Given the proximity of the specification and payment for these assets to Government, and the requirement for an implicit guarantee of financeability in RAB, it isn't clear that borrowing through RAB for such assets would be materially different from using Government's own borrowing and contracting robustly for construction and possibly maintenance activities.

In respect of assets where there is a strong element of user payment, for example directly tolled roads, there could be more of a role for a RAB financed "in perpetuity" model. One issue to be considered would be how to "seed" the RAB, or whether such a structure could be created from a zero-asset base.