To Anne Bernadette and Turnip the phrase you are looking for is "I say!"
concise, tautological and useful on any occasion with a slight change of cadence. Best done by James Hugh Calum Laurie, peculiarly enough.
Or Leslie Phillips
the master of the lascivious 'I..saaaay'

seemingly learned at the Italia Conti drama school, otherwise he would be saying 'Oi seh' like the other cockney boys. It says a great deal about Britain at the time that his ability to fake a posh accent meant he got into officer's training in WW2.