If you’ve visited Iran, Iraq, Syria, Sudan, Libya, Somalia, or Yeman since 01 March 2011 you are required to get a US Visa for your visit to the US. If you already have an ESTA you can check your ESTA status here . If you want more information you can contact the CBP information centre on the numbers provided on their website or visit the closest US embassy or consulate.
From the 1st of April 2016 you’ll need a biometric passport with an integrated chip (E-Passport) in order to enter the United States under the Visa Waiver Programme. British Passports issued from October 2006 already have this and should have a recognisable symbol on the front cover.