It is not always cut and dry with a UK citizen having a passport....
The US government also says if anyone has been convicted of any offense (including a parking ticket, traffice offense, speeding and upwards)
Which means you have had to have been taken to court and found guilty on something....you must get a visa stamped in your passport from the nearest American Embassy.
And like everyone else said it has to be machine readable (the maroon passports) and you fill out the little green card on the plane..NOT all airlines are included in the visa waiver scheme (little green card), I think all the major airlines are tho.
As of 4th Oct 05 a new requirement passengers into the US must arrive at the airport with full details of their address whilst in the US, this must include the zip code. The only exemptions are in-transit passengers who will be in the US for less than 8 hours.