Highly accurate currency charts are available from 1990 onwards at FXTop.com, with long-term exchange rates between the British Pound and U.S. Dollar listed as far back as 1840 on Wikipedia. There are a number of additional popular currency exchange websites including Yahoo! Finance and XE which list historical data.
Many important world currencies were introduced roughly 200 years ago, including the U.S. Dollar established in 1785, the Swiss Franc established in 1798 and the Mexican Peso established in 1823. More recent currencies include the Japanese Yen established in 1871 and the Euro established in 1999. Finally, the first recorded British Pound was established in 775, more than 1200 years ago.