Do you actually have a Visa for the US, or are you just visiting the US on holiday? If it is the latter, you can travel under the 'Visa Waiver Programme' if you are a UK citizen. You used to fill in a form on the flight which gets handed in when you land, and it gives you right to stay in the US for 6 months. Now ESTA is in place, you have to fill in the form before you fly and get 'passed' for travel. However, there is an overlap, and you still have to fill in the green Visa Waiver form and hand in when you land.
Hope this makes sense!