Tony Blair is taking 1600 troops out of Basra in the next few months and will aim to be down to only 3,000 or so (from 7,100 now) by the end of the year. Denmark is also going home.
This is a rout, there should be no mistake. The fractious Shiite militias and tribes of Iraq's South have made it impossible for the British to stay. They already left Sadr-controlled Maysan province, as well as sleepy Muthanna. They moved the British consulate to the airport because they couldn't protect it in Basra. They are taking mortar and rocket fire at their bases every night. Raiding militia HQs has not resulted in any permanent change in the situation. Basra is dominated by 4 paramilitaries, who are fighting turf wars with one another and with the Iraqi government over oil smuggling rights.
Blair is not leaving Basra because the British mission has been accomplished. He is leaving because he has concluded that it cannot be, and that if he tries any further it will completely sink the Labor Party, perhaps for decades to come.