Strategically though, what have we achieved?
Well that depends on your point of view and which of the various government lines to take on our involvement you actually want to look at.
If you want to consider our involvement in terms of defeating AQ and limiting Afghanistan's utility as a terrorist training base, then strategically we achieved most of that within a few short months of having gone in to Afghanistan. So quick and successful was the initial round up that I believe the USAF ran out of targets to bomb quite early on. However, taken from the perspective that an awful lot of the terrorist training facilities either moved over the border into Pakistan's tribal areas or belonged to groups other than AQ Core, then strategically, the fact that many of these groups are still running, that the Pakistani authorities still haven't got a grip on the tribal areas, nuclear weapons security remains a problem etc suggests to my relatively tiny mind that the broader area is still as insecure as it was in late 2001 / 2002 and thus success was once again more tactical than strategic
However, we didn't stop there - possibly spurred on by criticism that we didn't finish the job in Iraq in 91 and should have rolled on up to Baghdad - we can also take a view on Afghanistan from the perspective of nation building. Shortly after we had rolled up the majority of AQ Core's presence and capability, the Bonn Conference shifted focus to ensuring Afghanistan was developed into a peaceful and relatively prosperous society (they could be stinking rich if they wanted - think Saudi but with minerals rather than oil).
At the Bonn Conference, different countries took the lead for the different areas deemed essential to building the future Afghan nation. For its part, the UK took on the lead for counter-narcotics. Hmmm well that went well didn't it. And after much fanfare and publicity about CN being HMG's strategic aim, that quickly died the death and was never really heard about after probably about 2006/7 - roughly tying in with our insertion to Helmand and sudden surprise when the narco-bosses who had up to then been left alone suddenly felt threatened and put up a fight and violence levels suddenly and surprisingly sky rocketed.
Ever since then, when the lefties have cried out what are we doing there, the standard response has been ensuring security at the local level to enable the Afghan government to extend its capability to govern from the major cities and into the towns and rural areas. In doing this, it will deny the 'insurgents' the space they need to operate and in doing so will increase freedom of movement and stability and thus confidence amongst the populace. The only snag being, it's all well and good sorting people out at the local level if the government is still dysfunctional at the national level and can't stand the politicians or the stabilisation force trying to assist it at the strategic level.
And this is the crux of the current problem. Tactically we, in a broad coalition sense, have been largely successful and have managed to deny 'insurgents' the ability to operate in many local areas. But the politicians - Afghan, 'western' and NATO / ISAF leaders at the strategic level have such a dysfucntional political relationship that unless they can put aside their differences, they will never ever move Afghanistan from where it is now to where it could be in 20-30 years down the line if they really put their minds to it.