According to the United States Embassy in London, British citizens who meet certain requirements may be able to travel to the United States without a visa under the Visa Waiver program. However, if you are planning to work in the U.S., or stay longer than 90 days, you will need to apply for a visa.
Length and Purpose of Stay
British citizens may enter the United States without a visa if they are traveling for business or tourism and will not be staying longer than 90 days.
In order to take advantage of the Visa Waiver program, you must have a machine-readable or e-passport. If you are unsure whether your passport meets these requirements, visit the link in the Resources section for more information.
British citizens entering the U.S. without a visa must have a return ticket or itinerary. Be prepared to present it to immigration officials if they ask for it.
Long-Term Visa Requirements
If you will be staying in the United States for longer than 90 days for tourism or educational purposes, you will need a visa. In order to apply, you must make an appointment at a U.S. embassy or consulate in the UK.
If you plan to work in the United States, you must obtain a work visa. You must make an appointment with a U.S. embassy or consulate in the UK and you must already have an offer of employment from a company in the U.S.