In BrE, one can apparently use I'm sat here to mean I'm sitting here. This seems to be a relatively modern usage:
I had originally thought that this was a regional or dialectical variant and had asked a question about this, but the discussion in the comments and the fact that I found many occurrences of the phrase in print (searching Google Books) suggest that it is in fact quite widespread:
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]
When I'm sat beside his grave / The reason I'm sat here crying / Is for the life I couldn't save. [source]
I'm sat here, tears running down my face and no one asks if I'm okay. [source]
However, it was suggested in the comments (1, 2) that there may be a difference in meaning between I'm sat here and I am sitting here with the former beeing more negative. Perhaps because it implies a certain "passivity", that the person so sitting was placed there as opposed to having chosen to sit. While some of the examples I found and am quoting above do seem to be negative, I don't see evidence of such a trend.
So, my question here is i) is there actually any difference between I'm sat here and I'm sitting here and, ii) if so, does the former have some sort of negative connotation?
Please note that this is about the specific usage of sat to replace sitting and not for cases such as "I sat the baby down".
It is also not a duplicate of Is "I am sat" bad English? which is asking whether I'm sat is "good English". I am instead asking whether there is any subtle difference in usage between the two.
Best Answer
Ignoring the grammarian 'it is wrong' response, the 'standard' (for want of a better term) answer is that it is a quasi-passive. Fowler, for example, explains it as such in his Pocket Modern English Usage. The basic idea is that sentences such as "someone broke the car" and "the car needs fixing" are passive-like in function though not form as the actor is external and/or unknown.
Maybe more relevantly, the same idea allows the passive voice to have a continuum of function for the past participle from adjective-like to verb-like. In this, sentences like "I'm sat/I'm stood" are more adjective-like in function while "I'm sitting/I'm standing" are obviously more verb-like.
In short, "I'm sat here" is similar to "I'm big" or "I'm tall" - you are describing yourself more than saying what activity you are engaged in. In contrast "I'm sitting here" is saying what you are doing.
In the spirit of being fair, I am not 100% convinced by this explanation but neither can I think of a better one.