(Disclaimer: I know that £20 is a lot of money to a lot of people. I also know that some people probably feel they've already paid enough money for their Standard ticket, and I'm not saying that those people are dummies for not doing it.)
I remember the first time I heard about it, in the late 1990s, when it was just £15. I was living in The North and my friend Dom was a trainee solicitor in London and he'd come home and regale us all with tales of the exoticism of traveling first class. Weekend Foist, we called it, for little or no apparent reason. I don't think it's even called Weekend First now, so the name is even more tenuous.
When I moved to London, I'd use it on a Sunday any time I'd come home and I use it today whenever I'm back. I'm sitting in Foist Class now, in an empty, quiet carriage at 2pm on a Sunday, going from London to Wigan. My point is, I can't believe everyone I didn't except in the disclaimer who has £20 to spend and more than an hour on the train isn't doing it.
You arrive at the station. You find an unreserved seat in First Class (which is most of them). You plonk yourself down, pay £20 when the ticket machine person (CONDUCTOR, Paul, use your words) comes round and Bob's your mum's cool brother delivering you a host of free gifts.
To whit: Free wifi of mainly acceptable speed (£5 in Standard Class, it drops out occasionally but it does where I live in New Orleans, so nbd). Free tea, coffee and water (as much as you can drink, though I have never tested the upper limits, should they exist). Free food/meal pack: see the above picture: a nice tomato and egg sandwich, pretzels, a biscuit and a chocolate (two sandwich options, I BET you could ask for one of each). Free calm, civilised, roomy, comfortable carriage with actual places for luggage even if you turn up late, power points at every seat and a range of cultural lectures given on the hour. OK, not that last bit but it's (IMHO) more than a £20 world away from Standard. I mean, I have crowding issues, but still.
Anyway. Bully for me, I know. But it's a rare Shandy Pockets recommendation: Book early, get a cheap seat, make one of your travel days a weekend, and it more than pays for itself.
More info: www.virgintrains.co.uk