We must remember a British passport is no guarantee of a person. Richard Read the 'shoe bomber' was Brit, we have had several 'British citizens' held at Guantanamo- though from their names one can hardly believe they are 'British'. People are using the cover of a British passport to carry out anti-Western activities. No wonder a British passport is not regarded as particularly 'safe'.
There is a load of virulent anti-Americanism using this Immigration procedure as an excuse to let rip. We did not lose 2500 people and some expensive real estate less than 4 years ago- had a large slice of central London been taken out, we would not be happy bunnies either.