Resist the urge to buy foreign currency before your trip. Some tourists feel that they only have to have euros or British pounds in their pockets when they get off the plane, but they pay the price with inadequate exchange rates in the United States. Wait until you arrive to withdraw the money. Whatever you do, DO NOT USE these cards to guarantee your hotel or rental car bill or to buy gas at automatic pumps.
In these situations, an amount (which can be 200 or 300 pounds) will be withheld to guarantee payment of the bill. The problem is that, even if you don't spend that much money, it can take up to 30 days for those funds to be released. In the meantime, you can't use the money you've deposited on the card for the rest of the trip. Use your credit card for guarantees and then pay the bills with the prepaid card.
The security of having a few pounds as soon as you arrive in London is good, but make sure you always find the best place to exchange your money. In general, banks offer you worse exchange rates and charge you additional fees. It's safe to say that you should avoid buying foreign currency at banks, airport kiosks, and exchange offices, unless you're keeping foreign currency in cash and want to get rid of it. In that case, it's best to exchange enough money to survive until you find a more reasonable option, such as an ATM or bank.
Exchanging cash before you travel can be a minefield, as low rates and high fees are common. The ideal is to use a card with no foreign transaction fees to pay for purchases abroad as much as possible, so that you don't have to exchange cash or use a foreign currency while traveling. London is starting to pay electronically, so it's a good idea to have at least one card when you travel to the UK, but ideally two cards.
