Everyone from the UK going to the USA by air, even those just passing through, must fill out the Electronic System for Travel Authorisation (ESTA) in advance or be they'll be turned back.

Most British Citizen passport holders to visit for up to 90 days without a visa, but must get an authorisation via the Electronic System for Travel Authorisation (ESTA) prior to their journey.  Visitors should register through the ESTA website at esta.cbp.dhs.gov and are advised to do so at least 72 hours prior to travel.  If you do not have an ESTA you will be refused travel to the USA.  The US Customs and Border Protection department have announced that there will be a charge to apply for an ESTA from 8 September 2010. All payments must be made by credit card.