It could be as simple as British delays in implementing machine readable passports.
"The Patriot Act legislated that all Visa Waiver Program travelers must have a machine-readable passport. As of October 26, 2004, if a visa waiver traveler does not have a machine-readable passport (MRP), then the traveler must present a U.S. visa at the port of entry to enter the U.S. This includes all categories of passports -- regular, diplomatic, and official, when the traveler is seeking to enter the U.S. for business or tourist purposes, for a maximum of 90 days." http://travel.state.gov/visa/temp/without/without_1990.html
Thanks for the information. I wasn't aware of that!!!