Having debated all this with the Chief Inspector and NR some years ago it is worth recalling that it is up to NR to demonstrate that any new 3rd rail electrification is safe, ie. reduces risk so far as is reasonably practicable (as per the 1974 Act). It is not up to ORR or RSSB to do this, the former reviews risk assessments that might be done against this requirement during project authorisation and the latter devises standards in consulation with the industry. NR holds the safety case for the infrastructure and it is their responsibility to plan and build any new works to meet the 1974 Act requirement -- both in respect of the staff and the public.
In the case of extending 3rd rail electrification, the fundamental problem is the ability of staff and the public to, as a consequence, more easily come into contact with live conductors, bearing in mind in also the Electricity at Work Regulations which, although having been in place since 1989, were not given much priority by the industry until recently (ballast packing with shovels near live conductor rails, anyone?).
In essence, my understanding has been that ORR felt that it would be near impossible to make the case for 3rd rail extension compared either to OHLE which is (by design) lower risk or (because this is the starting point) non-electrified working where electrocution risk does not arise. And these days, it would be also be against battery or hydrogen traction, presumably... Third rail would add a quantum of risk which ultimately could only be partially (=not fully) mitigated by actions such as stern warning boards at eg crossing points/platform ends or enhanced staff training.
Therefore ORR had what seemed to be a quite well founded presumption that any new 3rd rail electrification (other than tiny new sections in eg. depots) would be unlikely to be consistent with the 1974 Act and 1989 Regulations as they were then being applied (ie. with the background of overall lowering of passenger and staff risks, certainly when compared with 1930s, 50s and 60s when most 3rd rail was put down.). This was amplified by the Chief Inspector's several fairly stark public pronouncements on the topic, such that NR felt that there was little point in attempting to develop risk assessment as ORR seemingly could never be convinced.
As pointed out up-thread, I think all that is being signalled here is that ORR is potentially ready to talk a bit more positively about standards or mitigations for 3rd rail than in the past -- but presumably within the current legal position (which is not going to change in a hurry). It may be that through measures such as -- enhanced fencing, switching on of higher risk conductor rail sections only when a train is approaching (much easier technically to do now), half-barriers on foot crossings (NS has a lot of these where passengers still cross tracks to/from platforms to reduce risk of being hit by trains), and procedures to isolate rail during maintenance/inspection ( = tracks under possession more often), tougher training and certification requirements for track workers and better detection of when conductor rails are live (LUL introduced warning lights some years back to provide a visual check on whether the juice was on) -- the risk could be mitigated sufficiently. It may also now be that, thanks to RIDDOR monitoring, NR has much higher quality data on the current risk, ie. injuries and fatalities on the current 3rd rail networ,k and can show the level of risk is actually very low (which it seems to be). But it's a tough call, particularly in respect of track workers and bearing in mind all that NR is doing at the moment to reduce the risks they face during patrolling and routine maintenance.
We may need to acknowledge that the 3rd rail system is inherently just too challenging to extend even on short bits like Basingstoke - Reading. It's a great story for Rail Magazine to run, certainly worth another examination of the issues, but don't hold one's breath.