You are dead right in your observation that Brit troops (or any troops for that matter) go where they are told to go and do not have a choice. Saying to your commanding officer "Sorry sir but I realy don't want to go to Afghanistan" doesn't realy work. Once you sign up you go where you are told no questions asked and you can't think like a civvy who can just walk out of a job they don't like. Try walking out of the British army and they send big hairy arsed MP's (with weapons) after you and then you get courtmatialed for desertion.
here in the states you can. they call these people concientious objecters. i call them pussies. when i was in we actually had some girl from my post say that she shouldnt have to go because her recruiter didnt tell her she may have to go to war. umm your in the army what do a think they do. but if you say its against your beleifs well you dont go.