BR suffered through the media. which led to much of the perception from the general public that it was awful. But many of those people had never been on a BR train, and were solely influenced by jokes about BR sandwiches and sausage rolls on TV, report sof broken down trains in the papers, and moaning commuters on the news (some things don't change).
Amongst those who actually used, for example, the HST services on the Western Region, BR was seen overall as quite good. The trains were comfortable, more spacious than now, faster than now, and were a huge advance on what had gone before.
Yes, there were good and bad services, and some were truly awful, but we are still in that situation now!
I think that's the central issue in this discussion. And not just in respect of new trains either, but also the other infrastructre required to run the system. just look at how much money has been put into station modernisation! If BR had received the funding that has been thrown at the railway since privatisation, then who knows what would have happened.
The decisions BR made in respect of cutting services, singling track and failing to invest in lines like Uckfield were solely due to the funding constraints imposed by various governments. While we may look back now and say some of those decisions were wrong, at the time the BR management really had very little choice.