You may not see it all the time but that is how they check the cards and allow you to add them, if they can't get a preauthorization, meaning there is no money on the card it won't be accepted
UK Entropay is different from US and other country, here you can't register your vcc card with any address. So if you r in UK best way to go sainsbury telco,whsmith etc and look for visa/mastercard gift card. It works fine.