Simon Jenkins wrote about this a few weeks ago. One thing that he said that really struck a chord with me, is that armed forces are a spectacularly expensive and ineffective method for fighting terrorism.
Armed forces have historically been the method that countries use to protect themselves against existential threats expulsions or killings of entire populations, the replacement of the government with an imposed rule by another, etc. Neither the Taleban nor Al Qaeda pose any meaningful existential threat to the UK there is no substantive risk that the British parliament is about to be replaced with an Islamist ruler, for example.
Even if they killed thousands or tens of thousands of us by launching some kind of horrific attack on the UK, they would not pose an existential threat to us. More than 99.9% of us would still be living. The economy could continue (once we'd got over our shock and assuming we didn't shut it down through over-reaction a la backscatter machines and groping of three year olds in airports, which in the US is on track to kill more people than the terrorists by shifting people from planes to cars, which are much more dangerous).
What's more, to the extent that we face serious and disruptive threat from Islamist terror, eg dirty bomb in London, gunmen in the House of Commons, cyberwar on the LSE, that kind of thing -- the armed forces can't help us. The things that help us are for example, effective intelligence services, effective cooperation with other states, and trying to harm the interests of others as little as possible.
I understand why people with loved ones in the armed forces get angry with posts on this topic, but that will not make me shy away from stating my view: what the armed forces are doing in Afghanistan is worse than ineffective in protecting the interests of the UK -- it's actually counterproductive, creating a greater likelihood of damage to the UK than if we'd done nothing.