I suspect that you'll get a better exchange rate in England, for the reason that currency exchange is much more common there than in the US (given its proximity to other nations with different currency). But changing physical currency is a bad idea anywhere, at least for more than say $50 to get you through the airport.
The reason is that you're dealing with a retail transaction. The exchange bureau sets its rates so that (1) it covers the costs of having a person present to physically exchange currency, and (2) it makes a profit even if the real exchange rates fluctuate (within reason).
You'll get the best rate when using a credit card, because the card transactions are cleared at the interbank rate. However, your card issuer may charge a foreign transaction fee (in the US, at least, you can find cards that do not charge this fee).
The second best rate is to use your ATM card to withdraw money from your personal bank account. Again, this is settled at the interbank rate, and again may incur a foreign transaction fee and/or bank fees. There are some international ATM agreements that reduce or eliminate these fees.
Bottom line: talk to your bank and credit card company. And change 50 pounds before getting on the plane.