Currency

There are ATMs all over the airport. That would be the most cost-effective way to get cash, using a debit card. Never use a credit card to obtain cash from an ATM, unless it's an extreme emergency. It is treated as a cash advance and starts accruing finance charges immediately (and the interest rate is higher).

Before you go tell your bank you will be traveling to the UK. You should ask what their foreign transaction fees are if any. Use a debit card at an ATM. Most UK and European ATMs don't add fees but some do, I understand travelex has been adding ATMs at airports that may charge fees. This info should be on the machine and there are many at Gatwick.

Yes, ATM's before leaving the airport is the best way to get started. Be sure you use a bank related ATM rather than an "Exchange" (look for bank in the name and avoid any that have EX or exchange in the name). I would use a debit card for cash. Your credit card will charge a fee plus interest from the date of the withdrawal. A debit card is better than an ATM card because the Visa or Mastercard logo will allow you to use virtually any ATM in Europe. Check with your bank on fees for foreign withdrawals. I had an account with Wachovia (now part of Wells Fargo), they charge an exhorbitant fee plus 3 percent. I opened an account with USAA federal savings bank who have a one percent fee on all foreign withdrawals, probably saved me $10 per transaction over Wachovia.

Brad,
I think you will find that ATM cards with a Plus or Cirrus logo will be accepted in most ATMs. See http://visa.via.infonow.net/locator/global/

Yes, apparently Plus is now related to Visa and Cirrus is related to Mastercard. If you don't have a debit card, it would be worth checking to see the availability of ATM machines that will take your card. Here's an article from independent traveler that might give you more information. http://www.independenttraveler.com/resources/article.cfm?AID=41&category=8