I had always associated the construct I'm sat here (as opposed to I'm sitting here) with the north of England. I know I've heard it from people with Yorkshire or Manchester accents, for example. Yet, I was recently speaking with a couple of Londoners, one of whom used it and the other stated it sounded natural.
Here are some examples I found in Google Books to clarify the specific usage I am referring to:
Don't think: I'm sat here waiting for my plays to be produced; think: I am sat here waiting to write those plays that can only be produced, now. [source]
I'm sat here in Vittles waiting for a second pot of tea, and life is OK, on the whole. [source]
I'm sat here, in the back of a van with my Thermos full of hot tea, protecting a car-park. [source]
And it'sonly now that I'm sat here to with Emma that the absurdity of what I'm doing is starting hit home. [source]
I'm sat here watching and listening to them talk. [source]
So, how common is this in the UK? Is it actually regional and, if so, of which region, or is it a more widespread expression?