If I want to travel from Birmingham to London I have a choice of 3 different train companies, as well as a coach company and driving (either hiring a car with or without driver, or using my own car). That's tons of competition, and it shows given the choice.
Great, now talk about literally any other line in Britain
London-Edinburgh (West coast vs east coast. West Coast is slower but cheaper)
Bristol-Manchester (XC via Birmingham vs ATW via Hereford - I usually take the slower cheaper Wales train as I prefer the route and the seating)
Manchester-Norwich (via Sheffield-Peterborough, Via London, Via Leeds-Doncaster)
London-Reading (GWML from Paddington vs South West from Waterloo)
Exeter-London (Paddington via Westbury on GWR vs Waterloo via Salisbury on SWT)
Manchester-Warrington (ATW via Newton, EMT via Birchwood)
London-Hastings (via Tunbridge Wells or via Haywards Heath)
And different competitors on the same track
Milton Keynes - London (West Coast, London Midland, Southern)
York - Newcastle (TPE, LNER, XC)
Manchester-Glasgow (Northern+WestCoast, TPE)
Stoke-Manchester (Northern vs XC vs West Coast)
That said, the WCML is extremely well run relative to the horrow shows in S and SE England.
If you live along the WCML (think Bucks, Coventry, etc), you have two providers: Avanti (formerly Virgin) and London Northwestern/West Midland (formerly Midland). You actually have Southern as well, but they are... not a credible provider.
HS2, if it's ever done, will further reduce pressure on the WCML and increase resilience.
But on the other hand, go up North and there used to be trains which are modified bus bodies[1]. Are those still there? So yes, absolutely, the rail network does need to improve across the country.
That means either higher tax to subsidise it, or it means higher prices on profitable parts of the network to cross-subsidise it
Turn up to any mainline station during peak hours and you can observe this for yourself (also non-peak hours in many cases). Trains which require reservation have scarcely any standing room.
Other countries would be quick to complain if you don't get the seat that you paid for.
When are you talking about? There was a lot of investment in the (laughably named) Thameslink 2000 program, which was put in place to dig the route out of the mess that the nationalised rail network had left it in by the 90s, it started in 2009 and is still undergoing improvements
When was it?
You suffered from the political decision that the government made to get rid of guards. No company would take it as a franchise because they knew the fight with the union would impact both profits and users, so they got GTR to operate it under a management basis.
This is now what is happening to every other TOC.
Southern is the reality of modern railway nationalisation, a little better than BR perhaps.
I think the privatisation could work if multiple companies could bid for a given journey / timetable slice, rather than having entire routes for themselves with no realistic way of removing them if the service goes bad.