Sorry but you only have to look at any govt quango or organisation to realise that they do not do efficiency. And like I said before I don't think the saying "a BR excuse" would have come out if BR was as rosy as you think it was.
Given that you seem to be the only one (other than your dad) to have heard of the phrase (all the more remarkable given the number of ex-BR employees on this forum), I wonder whether putting much weight on it is inadvisable? There were plenty of BR jokes in the media at the time (I'm old enough to remember the latter days of BR), mainly surrounding late trains and nasty buffet food, but, as has already been said, it was somewhat over-hyped, rather as today, the press makes out that every TV presenter in the '70s was a paedophile. No-one who recalls the BR era could seriously suggest that it was problem-free, but most of those were due to years of dodgy management being followed by years of chronic under-investment. I'd like to see evidence that there was a structural flaw with BR or that it was inevitably a failure due to being State-run.