worked in bookstores). And back in London, it seemed every bank
(branch) had an ATM-like machine outside for currency exchange -- I
put in $100 in $20 bills and it gave me the appropriate amount in
sterling (less a hefty fee).

These days, you just use the regular ATMs, since most people have ATM
or debit cards that can withdraw money from their home country bank
accounts. I still see occasional bill exchangers at airports.

R's,
John