This is a new experience for me because I am very much a prescriptivist, and have never accepted "native speakers say it" as an argument in favour of non-standard grammar. However, as an EFL teacher, I do have to explain to my students the things that they hear people say. The explanation I gave at the beginning of this thread is the one that I have developed for my students, and I confess that I have never put much intellectual effort into debating whether it's correct or not. People everywhere say it, and it makes grammatical sense to me, so I never really delved deeper. I acknowledge that reasonable people can disagree about what is acceptable and what isn't, but with all due respect to the fine nerds at wordreference, I will always consider my teaching colleagues to be the ultimate arbiters of grammar. Some of them are so erudite it hurts. If someone with forty years of English teaching experience says something is correct, then that's good enough for me.
So to address gottagettolondon's question:
It is not considered standard to put "stative" verbs (ie love, hate, like, own) in any kind of continuous/progressive form. These are verbs that describe states and not actions, so since states either are or they aren't these verbs can't be continuous. However, ever since McDonald's popularised "I'm loving it," people have increasingly been using these verbs in the continuous form. EFL textbooks now advise that stative verbs can be continuous if they express a passing state, one that is temporary and therefore can be thought of as an action. So you would still say "I love my husband," not "I am loving him," but in the event that love for your Big Mac overcomes you whilst you're eating it, you can say "I am loving this Big Mac." It means you are loving it right this minute, but aren't necessarily commenting on the permanent state of your feelings for Big Macs. Thus our understanding of states and actions has evolved.
The sat/sitting thing is an extension of this action/state issue. Saying "I was sat" suggests that the speaker considers sitting as a quick action that once finished becomes a state. It's not continuous because the action is finished and now we are describing the result of the action: that I am sat. Since using past participles as adjectives is a common standard practice, this makes perfect sense to me. Sitting doesn't involve any actual movement, after all. It's not the same as "I was playing," or "I was thinking," though think is a verb that can be both active and stative.
I'm out of time so can't elaborate further, but I hope this was a reasonably comprehensible explanation!
