Most UK passport holders gain entry into the USA via the Visa Waiver Program (VWP). You need a valid, machine readable or e-passport with an electronic chip.
See:
http://www.usembassy.org.uk/cons_new/vis…
Note: Failure to determine that your passport qualifies for the VWP may result in you being denied boarding by the airline.
Important: Some travelers may not be eligible to enter the United States visa free under the VWP. These include people who have been arrested, even if the arrest did not result in a criminal conviction, those with criminal records, (the Rehabilitation of Offenders Act does not apply to U.S. visa law), certain serious communicable illnesses, those who have been refused admission into, or have been deported from, the United States, or have previously overstayed on the VWP. Such travelers must apply for special restricted visas. If they attempt to travel without a visa, they may be refused entry into the United States.
Also note that having a visa or trying to gain entry via the VWP does not guarantee entry into the U.S.A
"After I have my visa, I will be able to enter the U.S., correct?
A visa does not guarantee entry into the U.S. A visa allows a foreign citizen to travel to the U.S. port-of-entry, and the Department of Homeland Security U.S. Customs and Border Protection (CBP) immigration inspector authorizes or denies admission to the United States."
http://travel.state.gov/visa/questions/q…