Visitors often underestimate the value of British coins. When you are are used to nickels, dimes and quarters, or the tiny five and 10 cent Euro coins, you can all too easily think of that pocketful of British coins you are clanking around with as just so much small change. A British pound coin may look like play money but it is worth about USD $1.65 ( in 2013) and two pound coins are worth more than $3. So a handful of coins could buy you at least a sandwich and a drink, maybe more - hardly small change for most of us.
And do spend it before you leave because banks and currency exchanges won't exchange coins into your own currency.