Liz, you said:
"Britches or breeches are the short, knee length trousers that were worn hundreds of years ago."

I always thought that 'britches' was an older name for long pants..

and then you said:
"Housecoats in the US seem to be what we would call dressing gowns or bedjackets"

I have always used the word "robes" for the down to the ankles or knee length garment that females or males wear over pajamas or to cover themselves after taking a shower etc.

???