The Canadians were certainly concerned about being vulnerable to attacks from below and quite a few of their heavy bombers were fitted with a 'scare gun' in the belly.
Unfortunately most of our heavy bombers simply did not have room for a proper lower turret,especially those a/c fitted with H2S.If the Canadians were aware of the risk I find it difficult to believe that our 'Airships' were not aware of the possibility of attack from below - as already posted - attacking from below was quite normal during WW1 and most RAF senior officers were WW1 veterans.