Why don't I wear one? Because David Cameron is just plain wrong when he says that:
The poppy appeal raises money for British Army veterans and their families. It doesn't restrict funding to those who fought in the one justified war the British have fought in past centuries (WW2, in case you're wondering). It represents one side in a string of conflicts, many of which can't be justified: Ireland over the centuries, India, Kenya, Malaya, South Africa, Suez, Iraq, Afghanistan (repeatedly) and a whole string of other vicious wars of conquest and oppression which can't be justified. Most of these people haven't given their lives for 'our freedom', but for colonialism, capitalism and racial supremacy. I can't see how 'our freedom' was served by invading Iraq, arming Saudi Arabia or bombing Libya: there are arguments for and against these actions, but no Saudis have been freed, let alone the rest of us.
The usual counter-claim to this is that we're helping and remembering the soldiers, who don't make policy. There's something to this: economic conditions drive people into the armed services very often, and I wouldn't blame individuals for that - though I do note that the UK Armed Forces are professionals: nobody's been conscripted. If the poppy symbolically represented all those who did what they were told without being responsible for wars' intentions, then we'd remember all those German conscripts too.
I will wear a poppy one day: when it represents all those killed in war, without regard to which side they were on, or whether they were military or civilians, such as the Bloody Sunday victims. It's time that public discourse on this grew up: the British state is not and rarely has been unequivocally supportive of freedom and democracy. Not every war has been a 'just' one.