As of July 2014, a twenty pence coin is worth around 34 U.S. cents. This amount will fluctuate depending on the currency exchange rates between the United States and the United Kingdom.
Due to the fact that the coins are still in circulation in the U.K. and they have only be in circulation since 1982, the coins are not worth much to collectors in the U.S. or the U.K. In some cases, twenty pence pieces that are older than 20 years may be worth slightly more than face value, however they are often not worth the time and effort it takes to sell them to a collector.
Since they are from the U.K., they are not considered legal tender in the U.S. and cannot be used to make purchases of any kind. The only way legal currency can be obtained from a twenty pence is to have it exchanged at a currency exchange center. Although they are considered legal tender in the U.K., they are only worth their face value and they can only be used to make purchases for amounts of 10 pounds and under.