Maybe you're not British. I'm not, and British weather is doing all it can to prevent me from spending time in the UK, and yet, I have more experience with £50 notes than you have. I haven't used a credit card this week, nor the week before.
I am indeed. Perhaps they're handed over more readily by foreign exchange bureaus. If I were to be given a wad of them I'd be looking to change them going on my own experiences. Twenties are ubiquitous. A fist full of fifties will invoke wonder in most Brits and they'll assume you're a scrap merchant or drug dealer.