Where can I exchange American Dollars for British pounds?

I'm going to London in February and want to transfer my money into british pounds now because over the last month the exchange rate has gotten much better. In case the U.S. dollar becomes much weaker again I want to convert my money now because I already booked my trip in England and have enough money but in jeopardy if the exchange rate becomes worse again. And also, if the exchange rate from U.S. dollars becomes much better from now until february could i can convert back into american dollars and then back into pounds?