Something is rotten in the state of Basra
You have got to read the timeline supplied/posited by Juan Cole on the Basra flare-up between the Brits and Shiite factions:
Two British undercover men seem to have seen something suspicious and intervened. But somehow they got involved in a firefight with Iraqi government police. The two Britons were slightly wounded and were captured by Iraqi police (which seems to be penetrated by the Badr Corps, the Sadrists and other Shiite paramilitaries.)
Then a Sadrist crowd tried to storm the jail where the two British special forces operatives were being held by the provincial government. The Shiite crowds appear to have intended to hold them as hostages to be traded for Fartusi et al.
It was at that point that the British tanks rolled against the jail.