The state of differing cross network policies/archaic systems and limited places make us the laughing stock of Europe,
Actually that's not the case, Switzerland has a superb public transport network, but France, for example, is pretty rubbish, away from their TGV network, and even then there are some very odd service patterns.
The Portsmouth – Cardiff services are being lengthened in the peak, to combat the overcrowding anyway, as you say it's a very well known problem. To be honest, whether or not you require a reservation there's a common sense element IMO. I wouldn't try and carry a massive piece of luggage on the Underground in rush hour, and I wouldn't try and take a bike on a service which is publicised widely for being very overcrowded.
But that's just me, despite working in transport I drive everywhere 🙂