Exchange rates will vary as you travel, and in tourist areas you will frequently see traders who specialize in converting currency, including American cash and euros into British pounds. Rates are often very clearly posted (look for the codes USD for U.S. dollar and GBP for Great British pound) and might seem competitive, but if you look at the small print or ask the money changer, you will find that there are associated fees. These fees are either a flat charge or a percentage of the money changed, and they can add up. Other places to exchange currencies that usually offer lower rates include local banks in London and many larger post offices. Some hotels also offer currency exchange on site.
You might exchange dollars to pounds in cash at your own bank or another local financial institution before leaving the U.S., although this can take some time in the case that the currency needs to be ordered. Airports generally host exchange bureaus as well. It's not advisable to carry very large amounts of cash on your person, but landing with enough cash available to get you through the first day or two can be convenient. This also gives you a little time to look into the best places to exchange more money before you run out of cash.