Just looking at the uPVC windows fitted 5 years ago to this 1930s house, to replace all the preexisting. It seems to vary. The window of this room is on the inner leaf, with vertical closure strips to cover the cavity gap. That's a brick wall. Some windows around the kitchen area are on what must be the outer leaf; these are rendered walls (I was going to say pebble dash but I'm not sure what that is.)
Then we have a new porch on the side of the house, built a couple of years ago. There, definitely on the outer leaf.