The argument for the UK's Defence capability must stretch beyond Afghanistan and Libya. The Arab Spring has brought a stark message to all that, regardless of whether or not you wish to help, hinder or otherwise, the World is rather unpredictable and the status quo of partnerships, alliances and 'special' relationships will not last forever. Arguing that the ~£40Bn Defence Budget may be better spent because Afghanistan (and Libya) are wasteful to country wracked with debt is tantamount to stating the obvious.
Igitur qui desiderat pacem, praeparet bellum
If we lose the only true insurance policy we have to defend our interests as a Sovereign nation then our ability to dictate those interests will be taken from us by force eventually.