I don't get the point of the pre paid cards. Just use your debit or credit card. You get the bank rate not the travel exchange rate so it's better and there are loads of cards that do no foreign transaction fee now. For cash pre order it online, delivered straight to your house get the best exchange rates, as someone said above Martin Lewis has a comparison site for it the more you change the better the rate.
But I hardly take any cash now, credit card is safer as your covered for fraud, theft and pretty much everywhere takes it and you get the best exchange rate.