A point seemingly missed in much early coverage of the UK withdrawal from the south (that I have seen, at least) is the potential impact of that withdrawal on the eventual withdrawal of US forces.
There already has been discussion of how vital the Baghdad-Kuwait supply line is for ongoing US operations, and this concern is well-founded. The south is not as has been portrayed in some upbeat UK and US official comments today. Southern Iraq is a very much troubled region where most localities are dominated by militias (sometimes rival militias), governance (to the extent governance linked to Baghdad exists at all beyond the symbolic in large areas) is tenuous, security forces are in most cases far more loyal to militias (often local, semi-autonomous militia elements) than legal authorities (such as the mayor of Basrah), criminality (including large-scale oil & fuel smuggling) is endemic, and low-level assassinations of the relatively few Sunni Arabs still present there is ongoing. When, late last year, British forces attempted to turn over a major base to the Iraqi military (and more bases are to be left behind as UK forces phase out), it was thoroughly looted.
Levels of overall violence are dramatically lower in the south only because of the area's relatively homogenous Shi'a population and its distance from Sunni Arab insurgent strongholds to the north, not significant advantages in governance and the deployment of security forces loyal to formal civil authority. As British forces gradually pull out, the south will likely fall deeper into misgovernance, militia domination and crime.
The current problem of resupply from Kuwait aside, when US forces pull out of Iraq (and this is a "when," not an "if," by anyone's definition), it could prove difficult to move large numbers of personnel and millions of tons of weapon systems, equipment and supplies through this volitile area (an otherwise preferred route). In addition, the seemingly inevitable damage to bases and airfields left behind by the British probably would further complicate the issue of withdrawal for the US.
If all this weren't enough, the current Baghdad surge (an iffy proposition to begin with) also may well be affected adversely. For example, Muqtada al-Sadr's Mahdi Army has a large presence in the south--even near-control in some neighborhoods and localities. To the extent Sadr's organization and its Mahdi Army are pressured in Baghdad, and with the British presence thinning in the south, many leaders and cadres can simply take refuge with even greater ease beyond the effective reach of US forces and what passes for a government in Baghdad for the duration of the surge.