The pound was the most popular forreign currency until 1945. Until that date the international trade was regulated by this currency value, where all the transactions have to be through England and it was done with that currency.
The end of the Second World War was the end of british control and so did the pound as reference currency. Bretton Woods arrangement established that American Dollar would be the currency able to regulate all the commercial transactions in the world.
Actually, the pound is falling its value. Euro introduction in 1999 made the pound lose ground in the principal economic decisions of the world. This situation has made British financial institutions look for a solution for this problem, and one of the measures that banks have taken is to reduce the cost of interest rates to the level of the European Union, and so, be able to compete with the raising Euro. In some years it would be very different for the pound to be as it used to be some years ago.