Her indoors is off shortly to the Far East for 4 months and has just invested in one of these pre pay cards. She looked into it very carefully and eventually got one from Fair FX which she said had the best reports. Her friends have used them outside Europe and says they were great. When I go back to Thailand next year I will also be taking one as the charge for withdrawing from an overseas ATM is only about 1€ compared to about £5-£6 with our banks. You can easily top up the card from your normal account using computer banking so you needn't keep a large amount in it. Within Europe I cannot comment from personal experience but I should imagine the savings will be as large. With regard to buying items I would read up their terms and conditions beccause I am unsure if you still get the +£100 protection you get with normal credit cards. Hope this helps.
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