....I prefer to make my judgments on what has been actually achieved, not on what's in anyone's manifesto.
But that is not what you are really arguing, is it? What has been actually achieved is that technology has moved on and some TOCs have taken advantage of it. You are arguing that BR would not, or could not, have taken advantage in the same way or to the same extent. As it happens, the evidence from the years immediately preceding the pre-privatisation freeze suggests quite cogently you are wrong.
For instance, I used to travel regularly on the East Coast, before and after electrification, and throughout the GNER era. I would say that comfort, reliability and customer service were all improved during the late BR era, and set the benchmark that GNER maintained (to their credit). It is widely acknowledged that other TOCs, including especially Virgin, took a lot longer to reach that benchmark level, which had been set by BR.
Though, as has been written before, to guess how a state-owned BR would have operated over the intervening years is ultimately futile, IMHO it is damaging to maintain, as you consistently do, that all improvements and investments are attributable to the TOCs and privatisation. That is grossly overstating the benefits of privatisation. and gets in the way of more rational assessment of what is right and what is wrong with the current system.