Everyone has their idea of how to make the railway more "efficient" and achieving decent "economies of scale" is behind Andrew Haines (& others) wish for Regional VI railways under a common banner but that doesn't quite sit well with the DfT's aim to keep things smaller and they then "have to" (!) retain direct control.
Delay attribution jobs were created by just turning the various Ops Clerks into either dobbers or query posts so, with the continuing need to investigate delays, there isn't a huge lot to be saved there. However, Schedule 4 could be largely switched off now and as the OA operators don't go for Type 1 and 2 compensation anyway, you can see their isn't really a commercial rationale for anything going forward into new structures apart from, perhaps, the most disruptive possessions. Also the larger you make the TOCs now, the less Schedule 8 arguments you have to deal with.
The commercial side is where the big growth industry in jobs has occurred since 1994. If decisions on advertising, products and fares was done more centrally, a whole swathe of posts could be cut out. ORCATS is a system that is dying anyway - most TOCs, if they don't get 100% on major flows, have long since headed off in the direction of dedicated revenue, both own account and through bi-lateral deals, to get more of the revenue cake - that is the prime aim of going dedicated and always has been, at least on the TOCs I've been at. You can code revenue 100% to Lennon through the trains service code and if you want differentiation, i.e. a through train to x, just change the TSC at the appropriate point.
You can get some saving in head offices but don't forget the scale effect - you might need more than one, the BR regions didn't just have one HQ building - places like York were awash with separate ER Regional offices.
Train planning is a big area to get savings in. TOC TPU's are sometimes quite large and that is where a lot of the TPU experience now is so splitting out MK and creating integrated Regional TPU's would probably benefit the railway in a number of ways. Again, if you remove TOC commercial gaming in current franchises, the intricate "what if" timetabling work also goes.
Everyone goes on about management on the ground but unless you take some of the decision making powers off them (and turn them into just glorified supervisors), you could easily get into overload situations so you have to look at both the breath and depth of the job before making a decision. Certainly if you combined roles further up the tree in a Regional situation, there are significant savings to be made.
Some query Regional structures because of border interfaces. It never bothered BR or the big 4 - you just set up simple joint arrangements to manage those aspects.