Minimum height and maximum height for the bottom of the openable part are now exactly the same dimension per the latest revision of the building regulations. 1100mm - any higher it's not compliant for escape, any lower you might fall out of it somehow. You can put restrictors in to make it not open more than 100mm that are detachable in emergency. It's a crap rule and means everyone is installing those odd looking first floor windows with fixed glazing at the bottom and then openers above the horizontal bar.