Great explanation.We have the same usage here in AE, it depends on who's saying it. It is colloquial, very casual.
In general, it means I am leaving - I am about to to be out of here, I am gone.
When my teenage cousins say it, it may means they are escaping before their mother discovers the mess they left in the kitchen. It is said while running for the door.
When I say it at work at 5 pm on Friday while walking towards the door, it means I am not stopping no matter who needs me, do not tell me I have a call on line 3, I am leaving, I am already gone, Bye bye!