A、Because some civil prisoners were planning to break the prison. B、Because they found a major Iraqi criminal in the building. C

British force struck the police building in Basra before dawn on Monday. They had information that an Iraqi major crime unit based in the building was planning to execute civil prisoners.
  Challenger tanks and warrior armoured vehicles took part in the assault and the British troops were backed up by the Iraqi army. British commanders said they came under rocket and machine gun fire as they approached the building. They responded with heavy machine gun fire and killed seven gunmen. Later that morning when the assault was over, royal engineers placed explosives around the building and demolished it.
  The British military authorities in Basra said they had planned to disband the police unit, which has been accused of murders, kidnappings and attacks on coalition forces. But they decided to attack the building instead after learning of the planned executions.