[physicsguy]

Traditionally everyone had sash windows ("sliding windows") or casement windows ("swing open") made from wood. Nowadays sash windows are very expensive so almost everyone has casement windows, or even casement windows that look like sash windows, both constructed from UPVC. It's only in historic buildings people typically still install sash windows. I had to do that in my house because of it being a "conservation area" and it cost £14k for four timber double glazed sash windows! There are some UPVC sash windows now but they're pretty uncommon still.

[asdff]

I actually have a casement AC window unit. Not sure if this is the same dimensions as a UK casement window. It is as if you took a window AC unit and made it vertical orientation. My window still slides up and blocks the remaining void, but I imagine it would work just fine if it swung out and you built a plexiglass or plywood cover. Maybe you can also unscrew the swinging window and put it away in a closet.

The AC unit is a little different than "traditional" window units. There is no slot or anything really to grab on to the window or the sill. It is mainly secured by a brace that is also not screwed into anything really, just directing the weight of the external portion of the unit into the side of the exterior wall.