From the U.S. Embassy in London:
[Citizens of] the United Kingdom may travel visa free under the Visa Waiver Program if they meet ALL of the following requirements:
The traveler is a citizen of one of the countries named above, traveling on a valid, individual machine readable passport. Note: A passport indicating that the bearer is a British Subject, British Dependent Territories Citizen, British Overseas Citizen, British National (Overseas) Citizen, or British Protected Person does not qualify for travel without a visa. A passport which states holder has Right of Abode or indefinite leave to remain in the United Kingdom does not qualify for visa free travel;
Traveling for business, pleasure or transit only;
Staying in the United States for 90 days or less;
Plus, if entering the United States by air or sea is,
Holding a return or onward ticket. If traveling on an electronic ticket, a copy of the itinerary must be carried for presentation to U.S. immigration at the port of entry. Note: Travelers with onward tickets terminating in Mexico, Canada, Bermuda or the Caribbean Islands must be legal permanent residents of these areas;
Entering the United States aboard an air or sea carrier that has agreed to participate in the program. This includes aircraft of a U.S. corporation that has entered into an agreement with the Department of Homeland Security to carry passengers under the Visa Waiver Program. Note: Other private or official aircraft or vessels do not meet this requirement; and
In possession of a completed form I-94W, obtainable from airline and shipping companies;
Or, if entering the United States by land from Canada or Mexico,
Is in possession of a completed form I-94W, issued by the immigration authorities at the port of entry, and a $6.00 fee, payable only in U.S. dollars.
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. http://www.usembassy.org.uk/cons_new/visa/niv/vwp.html
Hope this helps.