Reference:
If we abandoned them now, they won't have peace.
Probably. Strictly speaking it's not our 'war' though, it's a UN mission to rescue a failed state. The UK does not need to be involved necessarily to the extent we are; we're there because of Nato commitments through ISAF. I don't know how effective UNAMA is but Afghanistan won't stop being a failed state solely due to ISAF actions. The UN has other resources to call on. In fact, resolution 1833 (2008) ran out a couple of weeks ago, we're on resolution 1890 (2009) now and that calls on other member states to contribute towards ISAF's efforts.