We will be landing at Heathrow and need to exchange some money, where is the best place to do that? Thank you, Nick
Don't exchange anything. Use a bank ATM just like at home (you should be able to locate one on an airport map).
If you actually have a need to rather than choosing not to use an ATM etc, then get a rate from an online currency specialist and take it into their shop; there are plenty in London. Note that the selling rate (into £) is not as close to the interbank rate as the buying rate. Even so with dollars you should be able to get within 1-2%.
Found this handy tool to locate Non Fee ATM's outside of Heathrow.
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Found several all over the area.
No bank-associated ATM will have fees. Any fees that are charged with come from YOUR bank. Do you know if your bank charges a foreign currency conversion fee or foreign use fee?