A nation-wide (monopolistic) service -- railways aren't (and can't really) be run according to market principles, why should anyone be allowed to profit from this?
No idea what it used to be like, but the current railways are beyond a joke. Just go anywhere into central europe and you'll notice a world of difference.
Most of Central Europe has more competition in the railway market than in the UK, not less! Re-instating a nationalized monopoly will just go back to the money-sink BR used to be (where, for example, kitchen cars remained fairly widespread on trains long beyond them getting much custom, because the unions wouldn't let them be dropped).
The problem with the current setup is that of the difference between freight and passenger train services --- move to running passenger train operating companies (TOCs) as freight ones are run, and suddenly we'll have a system close to most of Central Europe. When it comes to freight trains any competent person can get a license to be a TOC (this is not dissimilar to running public buses!) and then it's just a matter of drumming up custom and purchasing track access rights from Network Rail. The problem is the temporary (but long enough to be harmful!) monopolies private companies are granted as a result of the passenger franchise bidding competitions, nothing else.
What several other countries did was split up the incumbent as per EU regulation (there's nothing that diabolical about this), but keep the state incumbent passenger service (often with a division between local and intercity trains) while opening up track usage rights to competition. If a private company wants to come in and compete with the state incumbent --- go right ahead! We shouldn't forbid that, as the competition (at least in Central Europe) has forced the monopolistic incumbent to stay on its feet, and keep improving its service.
And you say they can't be run to market-principles --- for a lot of people, they can choose a half hour later train if it means they get a cheaper (and possibly better) service. If you look up trains between London and Gatwick Airport, for example, you'll see multiple companies running with a fair price difference between them. How is that competition not helping the consumer?