Hello David,
Chris is right by recommending you a Cashcard and as stated can be obtained at the Post office. If you google prepaid cashcards there are infact numerous ones on line offering various rates etc. For my JY holiday last year when various different currencies were needed I used a "CAXTON FX World traveller card". You load this card with GBP and when using this card payment made for cash / purchases is done in the local exchange rate for the country you are in. Caxton also offer prepaid cards in Dollars and Euros.
The choice is yours. I had no problems with my Caxton FX card and instructions are easy to follow.
Regards
Eric.