You are too old for the draft I presume.
I am not a pacifist but most of the military actions taken by the UK government can hardly be called defence.
Observation in recent war zones, eg Bosnia are deployments which are more neutral in nature.
Depends how and why they are deployed, prior to WWII the cabinet post now called the Minister of Defence was the Secretary of State for War a more honest title. The 1940 Churchill cabinet first used the title Minister for Defence.
It's OK to have principles when people you don't know are being maimed and killed but the possibility of personal involvement should sober a few up, one would hope.