That's interesting Simplyesl because I think there's the same explanation for both the points you raise.
Yes, Brits do say Are you finished ?
(as well as, not instead of Have you finished?
). By using it like this we turn it from a past participle into an adjective which describes a state rather than an action. (Compare it with : Are you ready?
) This often happens with other past participles (tired, interested, excited
etc) which are all used with BE to describe state. In other words, if we wanted to focus on the action we'd use it as a verb Have you finished?
but using it with the verb BE we're talking about the person's state of being finished with the task, and not what they actually did.
And I think the same thing is happening with I was stood/sat ...
Instead of describing it as an action, we adjectivalise the past participle and see it as a state. Interestingly this is exactly how other languages conceptualise it. French :J'étais assis
Italian : ero seduto
both mean literally I was sat
PS: You know we can also say I was just stood standing there
? But I don't think I want to get into that one ...