You can use your debit card for purchases, but there may be a transaction fee or currency conversion fee for every transaction, although it probably won't be that much - a couple of percent of the transaction amount, probably. I believe that if you have a Bank of America ATM card, you can withdraw cash from Barclays ATM machines at no charge. Just be aware though that because UK cards must all have chip and pin now, you may be told in stores that they won't accept your card because it doesn't have chip and pin. However, as many foreign cards do not have chip and pin, there should be an option for them to process a foreign card, it's just that the cashiers will probably have been told that they cannot do it (so if you're told you can't pay with it, I would ask them to check again for you).
Looking at my UK bank website, if I use my UK debit card to pay for something abroad, I will get charged £1.25 for every transaction. Alternatively, if I withdraw money from an ATM, I will be charged 2% for every withdrawal (min. £2, max £5).