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.
Money laundering. Not even joking. Large denominations of cash is one of the more important ways of transferring value across borders for laundering purposes, which is why the largest number of bills in the most significant international currencies (USD and EUR, primarily) are in denominations mich larger than their day to day use value should suggest.