I agree, that the US and British armies usually acted with restraint, partly because they found that the right thing to do, but also because they were usually well fed and found it in their best interest to keep the local population on their side. If they were hungry and isolated in a foreign land, that mask of civility might have slipped rather quickly.
For an example of a lapse in the professional British soldiers' code, please see the actions of British troops in Ireland in 1798. The burning, abuse of civilians, and general standard of violence in those days is every bit as horrid as some of the French activity in Spain. Its worst was short lived, but it was wholly inexcusable if one believes that the British were above that type of behavior.
Best......Mark