On the GN side we of course have an 8-car diagram on the Cambridge to Brighton service, which just happens to hit the peak flow going into and coming out of London. So peak passengers don’t need unplanned short forms, turn up at a particular time and it will be an 8-car 100% of the time!
It doesn’t seem uncommon for 8-cars to turn up at weekends during altered working, either planned or otherwise, on what are theoretically 12-car routes.
Apart from the single planned 8-car diagram, I find the other 10 (of 11) Cambridge-Brighton diagrams are quite dependably 12-cars on weekdays.
And the 12-cars on Saturdays are quite helpful in avoiding the crushed - even at 12-car length - King's Lynn/Ely fasts on a Saturday morning.