The origin seems to be Hulloo, from hunting as in the ‘folk’ song “Peel’s ‘view hulloo’ would awaken the dead or the fox from his lair in the morning”. Where that came from is anybody’s guess but might be connected with Hullabaloo. Then there’s the famous police “'Allo 'allo 'allo, what’s agoing on 'ere then?”, complete with comic knee bend.