Well from what I have read on reviews of these cash passports they don't sound that great.
Obviously u load it in the UK with money, so you might lose out on a better rate in resort.
Some cash passports I have read about also charge when you withdraw from the atm so again your Exchange rate in essence drops again.
I asked the same question on another forum before I found this amazing forum, and the general consensus was to either get a nationwide flexi card account, bring cash, or, and my chosen method, is travellers cheques.
They are insured and replaced within 24 hours.
Hope this is of help and no doubt someone with experience of using these will be along soon

Chris