http://www.planningportal.gov.uk/per...s/doorswindows

"When replacing any window, the opening should be sized to provide at
least the same potential for escape as the window it replaces."

That does say "should" rather than "must" ...

It does not open onto anthing - other than the ground. Two berooms open
onto a sloping roof. They open as an escape.

The height of the landing window above the turn in the stairs would make it
a difficult choice for escape. Fove other windows give a better option as
you could get up onto a bed to climb out.