ATM’s in Britain do not withdraw that denomination (they max out at £20), unless for some reason an individual wants a £50 note: they would have to visit a bank requesting for that (banks stockpile bills of all denominations). However, there is a stigma with a £50 note (money laundering, drugs, etc.) as often depicted in news stories about criminals caught with “dirty money” often have £50.
Even if you were to spend it at a store: they are like “sorry, I have to get my manager” to verify it’s authenticity, due to concerns of counterfeit despite the new bank note series being printed from polymer. I’ve heard that is often reserved for business related or larger transactions at certain stores (electronics, luxury brands) where they may be inclined to accept £50 in multiple quantities.
Barely used doesn’t mean useless
They’re not for regular people making regular transactions, it’s kinda intended for high value transactions
Wealthy people selling things (semi-)privately to each other is one obvious one, things like those £50k watches. Sometimes these wealthy people want to do their spending with utmost discretion, so cash is king.
Another is pawnbrokers, private currency exchanges and similar kinds of business that just have to deal with a lot more cash than is typical.
Don’t get me wrong, its usage will definitely be in decline, but I don’t see them taking it out of rotation any sooner than the fiver