It's probably difficult to find something to buy for £5 around Covent Garden, and no problem to spend £50. I'd much prefer some £50 notes than an empty cash machine.
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.
