That's true, the one place I see them most often is being smuggled out of China. Actually I have gotten them from cash machines in Covent Garden before as well. Since then I've wondered what criteria they use to pick denominations for each area.
There's also a high cash demand from tourists, and a high nighttime demand (when machines won't be refilled).
> smuggled out of China
That's unfair: completely honest tourists often have £50 notes, regardless of where they come from. The currency exchange office prefers high-ish denominations of most currencies. They take up less space, which matters when everything has to fit in a safe, and are generally in better condition.
Anyway, the relevance to our conversation: since value density is highest with £50 notes, that's what they tend to bring.