Not much has changed on the surface in Al Amarah, capital of Maysan province. It is still a wild and lawless place, close to the Iranian border, which Saddam Hussein never tamed. But in Iraq’s new democracy it harbours a new threat — Shi’ite militias, many of them allied to political parties. They vie with each other for power, with British forces holding the ring and often bearing the brunt of casualties.

Maysan is earmarked as one of the first of the four provinces UK troops will soon begin to leave. In Al Amarah three weeks ago I met Captain Richard Holmes, of the Parachute Regiment, in the joint British/Iraqi operations centre. “Most people are welcoming and friendly,” Captain Holmes assured me as we walked down the town’s main street. He showed me weapons collected by the Iraqi civil guard the day before — a rocket and an anti-tank mine. “We’ve found they have not been directed towards us.”

His words would soon come back to haunt me. Later that morning the captain and another soldier in his unit, Private Lee Ellis, were killed by a roadside bomb as they travelled back to base. Local people threw stones and petrol bombs at soldiers recovering the bodies.

But the deaths would not be allowed to affect the British policy of handing over the reins to local authorities, even though this sometimes requires them to negotiate with people closely allied to the militias. There is too much at stake to allow British casualties to affect the process of handover. The British believe they have gone as far as they can in Maysan, and that it is in “good enough” shape, as one British army officer put it, to enable troops to stick to their plan to pull out in the not-too-distant future.

Basra province — dominated by Basra, the country’s second largest city — will be the last area to be vacated by the British, who still have 8,000 troops in southern Iraq. It is very different here from Baghdad. The attitude among Basra’s Shi’ite population to UK forces is still tolerant, though far from the jubilant welcome I witnessed as the troops arrived three years ago.

Soldiers still wear soft hats on daytime foot patrol, but after dark it’s strictly helmets. Brigadier Patrick Marriott of the 7th Armoured Brigade, which helped to take the city in 2003, believes the enemy encompasses Shi’ite militiamen, tribal factions and people who “just want to poke us in the eye” after three years of what they see as occupation by British forces.

Basra is not like other parts of Iraq — most notably the Sunni triangle, which includes Baghdad and has been convulsed by sectarian violence. When the al-Askari mosque, a key Shi’ite shrine in Samarra, was attacked by Sunni insurgents last month, thousands marched in protest outside a British base in central Basra as a wave of revenge killings swept the country.

Three years ago such a mass of people would have brought soldiers on the streets, but the order was to stay on base and keep a low profile. The crowd passed angrily but peacefully.

The key to British withdrawal from Basra will be the state of the police force in the town. Hated and feared in Saddam’s time, the coalition is forging a new cadre of police officers. But corruption and tribal loyalties dog the process.

The provincial council stopped co-operating with the British in the wake of arrests earlier this year of what the British say are terrorist elements in the police.

This boycott, which many view as a flexing of political muscles by the new council, has slowed the process of mentoring local security forces which is now the main task of British military units. At the police training academy British officers, including ex-RUC men from Northern Ireland, put new recruits through their paces — 20,000 so far and 24,000 still to complete training. “They have a long way to go,” said Ian Elder, a retired police officer from Edinburgh. “We never did think we could change the tribal culture in the time we have here . . . it will take 30 years.”

Local polling suggests 60% of Iraqis in Basra believe that it will be the British who guarantee their security for some time to come. “I’m not saying I like the British,” said Haidar, a poor Iraqi squatting in an old Ba’ath party villa, “but as long as our government is weak their presence here is necessary.”

The streets still run with sewage, though consumer goods have flooded a city so long starved of them by sanctions. Brigadier Marriott believes Iraqi disappointment at the slow pace of improvements has changed the mood of the people.

For the British government nothing short of civil war in Iraq is likely to halt the process of withdrawal, although no timetable has been set. Last week came the announcement that there will be 800 fewer troops in Iraq by May.

But still over 7,000 will remain. British military officers are all too aware the pace and shape of the final pull-out from Iraq will depend on America. As Iraq teeters on the edge of a sectarian war, with its own security forces still weak and its politicians wrangling over the formation of a national government, Britain’s exit strategy from the south is far from certain.

Jane Corbin’s Panorama: Bringing Our Boys Home? will be shown tonight at 10.15pm on BBC1