Let's be clear, I'm saying nothing about the Big 4 in comparison - my comments are solely about BR and BTC.Yes, it is fair to criticise some of the mistakes that early BR made, but rather deluded to think that everything would have been lovely had the 'Big Four' survived. The LNER was effectively bankrupt in 1948 and the other three would have gone the same way by the early '60s at the latest. They would have followed the familiar U.S.A. pattern of mass closures, deteriorating infrastructure and no money for capital investment. Unlike the U.S., freight wouldn't have saved them either, for once the motorway network developed in the U.K. wagon-load traffic was doomed.
In a desperate attempt to stay afloat, we might even had seen the demolition of Paddington or St. Pancras, like Penn NYC, for a 'Madison Square Garden' type development - or perhaps diversification into airlines and tourism. Clearly government intervention would have to had to come at some point if any semblance of a national rail network was to be retained - even the U.S. created Conrail and Amtrak.
It is all very well getting misty-eyed about the 'Big Four' in the inter-war years, but apart from the few 'glamour' services much of the network was pretty dire with clapped out rolling stock, run-down stations and in dire need of investment. Yes, the Southern invested in electrification and the GWR in its major stations and the Devon and Cornwall holiday route, but mostly at government expense through grants and cheap loans. The LMS was particularly turgid with a poor motive power policy in the early years and eschewed electrification - it was a reluctant partner with the LNER in the Altrincham scheme and the Wirral scheme didn't come along until 1938. It did very little to eliminate its pre-grouping legacy of duplicate lines, stations and depots in Scotland, the North West, Yorkshire and elsewhere. The LNER was hamstrung from the start, with the collapse of heavy industry in the North East and was also stuck with servicing the Great Central debt.
However, I do think that the simple fact of nationalisation inhibited commercial and engineering innovation during that period, and that it is unreasonable to assume the same outcome whether nationalised or not.