I am somewhat sure that in the WW1 era, a Lord in service would salute a superior officer. But what was the case for eras when Lords had more political or temporal power? For example, would they salute a commoner appointed by their king as a general in charge of them?
By WW1 the officer class of the British military was mostly made up of people from the upper classes, not just aristocrats. A young aristocratic Lt. would have had to salute and "sir" his superior officers. Out side of the military (and his service being over) other rules apply. But while wearing the uniform his duties as a soldier would override any class privilege. At least somewhat.