We highly recommend Ace-FX Bureau de Change! We chose to use them as they gave us the best rate of exchange out of all the online foreign currency services.
Obviously, we were dubious about paying money to a company unknown to us (we don't live in London), but as we were only changing a relatively small amount we thought we'd try them out.
We ordered online, followed their simple payment method and within two hours of depositing the money into their bank account we received an email saying our currency had been despatched. The currency arrived this morning, not even 24 hours after our payment was made.
We will definitely be using Ace-FX again in the future.