In many countries (particularly developing countries), the best way to get a good exchange rate on the local currency is to exchange $100 USD dollar bills - and $100 dollar bills get a substantially better exchange rate from local traders than lower denominations of bank note.
But most banks and high street currency exchange places in London give dollars in mixed denomination pre-packed bundles (unless you preorder in advance), so if you're buying dollars specifically for exchange abroad, most of what you get either won't be accepted for exchange or will get a lower rate.
Is there anywhere in London I can get hundred dollar bills over the counter?