the post office offer a credit card with no foreign exchange fees, meaning you get a much better exchange rate. for example, I bought my $ notes for $1.94, but got $2 to the � when using the Credit Card.
I found it easier to leave some cash at home, pay for all our meals, park tickets etc with the card (getting the better exchange rate) then simply paid the credit card bill off with the cash I left behind when I got home.
The PO also offer a Money Card, which is free to get, but you have to load at least �50 worth of $ onto it.
Bear in mind that most travel insurance policies only cover you for �250 worth of cash in you have a claim, so cheques/cards are a better option.