Some replies are focused on people not using notes anymore or only using them for small transactions of a few £... that's somewhat insular, they're clearly still used by some people (lack of use is more a reason to not print as many notes and not a good argument for not having a larger denomination note), that you personally can't envisage situaitons other than your own is moot. As for small transacitons of a few £s at most, why bother with £50, £20 or £10 notes in that case... just have coins and £5 notes right? But again that's not true for all and is more a reason to not print as many notes in general rather than not accounting for inflation.
Scotland and NI have £100 notes, the US has $100 bills and has done for many years (inc when it exceeded the worth of £100 today), the Eurozone not only has a €100 note but also €200 and €500 notes too.
Our money is worth a lot less than it was a decade or two ago but the notes issued by the BOE simply haven't kept up, aside from the £1 note being phased out we have the same range of notes as we did in 1981 but all of those notes are now worth substantially less.
On a related note... I'd also be in favour of phasing out 1p and 2p coins... the 1/2p coins went but now 1p and 2p is worth so little that it's pointless to have them, make retail price things in multiples of 5p.
I'd have preferred to see the £50 being more commonly used. It's stupid that people are suspicious of it and it feels like a novelty to hold one for the first time.
The €50 note is just as common as any other notes.
Most of it's naivety or hangovers from the 1990s and indeed pre-polymer notes. Given the price of a pint or even a bottle in London these days then paying for a round with a few friends can easily involve multiple notes if paying cash... it's less of an issue these days as the contactless limit has increased to £100, pre pandemic the contactless limit was only £30 so notes were more convenient. In some places they still are really as there's maybe some naff card machine that takes ages or one between a few bar staff etc..
The £50 essentially is the old £20 these days which lots of people still aren't quite used to.