What happens if you buy something in dollars when your own currency is pounds?

Ther is a system called the EXCHNAGE RATE. This allows people to work out as in your case how many UK pounds equals a Dollar.

When you pay for something in the US and you live in the UK they will work out how many UK pound is equal to the dollar price you want to pay. As of today (10/09/2010) 1 pound equals 1.52 dollars

so if a item is bought for 10 dollars actually cost you 6.5 UK pounds