It doesn't even sound like bad grammar to me. Would you say He was perched on a ledge or She was slouched in a settee? Or are they UK English as well?
Excellent question! I've dug pretty deep and still can't quite put my finger on it!
However, here are a couple of links that might help explain why it's wrong:
http://www.eng-lang.co.uk/grammar.htm -- scroll down to the entry for 'Sit Sitting', where the verb sit is equated in form to the verb 'walk'
http://painintheenglish.com/case/4796
My best guess (and I'm willing to be corrected by someone who knows more) is that 'was sitting' (similar to 'was walking') is the correct form of the past participle when used in the context of one's body movement. For example, 'The dog was walked this afternoon' is fine (if passive and lame...it's still grammatically correct), because someone else walked the dog; but you'd never say, 'I was walked down the street' if you were just walking (as opposed to being walked like a dog
).
It might have to do with transitive/instrasitive usage? Maybe combined with reflexive usage? In essence, 'I was sat on the sofa' really means someone else sat me there; whereas 'I sat on the sofa' and 'I was sitting on the sofa' mean I sat myself there.
'Was sat' is fine for dialect, but it's not (ahem, currently
) correct for strictly formal usage. I might have a character say it, but I would never use it in a formal context, such as in a publication for my day job as a tech writer, a formal business letter, an academic paper, a formal speech, and so on.
