Second for Chase downtown. Had to get Chilean pesos the other day. Said it would take 2-3 business days for them to be delivered but they arrived the following business day.
Also, the cheapest way to get foreign currency is at your bank followed by activating your debit card in the country you're visiting and pulling cash straight from the ATM.
All of the money changing spots charge crazy premiums.
And remember when you're in a foreign country using a card, always choose the option to pay in whatever currency they use and not to pay in USD. You'll save yourself money.
Most important thing, make sure your bank knows you are going to use your debit card in England. If you don't the machine will suck your card in and not give it back. It was a hard lesson to learn.
Get a free Schwab account that pays any atm fee and conversion fee and get money at airport atm when you arrive. I've been using one for a couple years for foreign travel and it's great. Bonus is you get a brokerage account as well.