Just a small correction - 'Be my guest' which also brings to mind another 'By all means'.
My dad and I are kinda childish and like to say 'Have at ye' because it sounds like old english.
Me - "Can I use your phone?"
My dad - "Sure! Have at ye!"
I don't recommend this because it's probably wrong and it sounds like something from a Monty Python skit... or one step below 'informal'