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
Great, now talk about literally any other line in Britain