1) I would definitely get $50 worth of pounds before entering Britain. This
way you won't have to stand on long lines at the foreign exchange counter
or the ATM machines.
2) I lived in France earlier this year and used my American bank debit card all over, and in Britain, Spain, Portugal and Italy, without any problems. However, you will need to have a card with a four-digit PIN. I only had one bad experience, when one day all the ATMs in a small Portuguese village were down.
3) If you don't trust technology, bring along some travellers checks. Since you will visit several countries, you might just have them in dollar denominations; having them in the various local currencies might be a bit unweildy. Still, I join those who endorse using ATMs, which are as commonplace as they are in the States, plus the foreign banks generally don't charge you a transaction fee (your home bank is another matter).
4) Use your credit card for shopping and dining wherever credit cards are accepted. You get the best exchange rate with credit cards. Visa and Mastercard are common in the countries you'll be visiting.
Hope this helps.
Evan in DC