You can't just quit the army, it takes at least a year assuming you've done your minimum period and you can still be sent anywhere in the 1st part of that last year.
Some deployments are very short notice, you could arrive back at work monday morning and fly tuesday, that doesn't give you much time to object your way out of doing the job you agreed to do.
Its never implied when you join that you can pick and choose the wars you want to fight, you just fight where your told. You trust that the British people will choose the right government that will send you to fight wars for the right reasons. Don't blame a soldier for fighting a right or wrong war - the people chose the government and the government chose to fight, the responsibility remains with the public.
Most of the UK public do support soldiers (they might not support the war) and their support is greatly appreciated.
If you feel the need to speak poorly of our soldiers try to remember that if we were been invaded it would be the same soldiers you speak ill of defending you in spite of what you may have said.