Milliband didn't literally mean to launch from Afghanistan onto the streets of Britain.
He means we should deny them a base and safe haven from which to plan and instigate its operations.
I am not convinced. If we do removed al qaeda/taleban from Afghanistan, they will not simply disappear, they will adapt. All the terrorists attacks on Britain occurred after we had gone to war. By Milliband's reckoning it should have prevented the likelihood of us coming under attack, instead, it clearly defines us as their No.2 enemy and increases the threat of attack.
Even the Russians got out after ten years, I fear that will be there for twenty, thirty years. I am not convinced al qaeda can be beaten this way.