BR is far enough in history now for a lot of rose-tinted cr@p to be spouted about it. I grew up with BR and witnessed the privatisation of the railways in my 50s. Here's few of my observations:
1) First, the biggest point: BR was always at the mercy of the Treasury for investment (or 'subsidy' as it's called for rail, as opposed to road). There was never any real investment so the network just got more and more run down. It was in a death spiral, with lines being closed, or singled, junctions and signalling taken out, and life-expired stock pushed into extra use, all to 'cut the cloth to fit'. Heck, BR even misrepresented the repair costs of Ribblehead Viaduct to try to get the Settle Carlisle closed! It could not carry on like that for much longer; hence privatisation, which allowed far more investment to flow into rail.
That wasn't the case everywhere though. I started catching trains in the late 80s in West Yorkshire. Between 1986 and privatisation station (re)openings happened nearly every year. The train service between Halifax and Bradford/Leeds improved from 3tp2h to 4tph. Admittedly this was with funding from the PTE. However since privatisation only 2 stations have opened with another 2 due by 2014.