To the best of my knowledge Building Regs state:

Requirement B1- means of escape in fire

All windows to habitable rooms (but not kitchens, utility rooms, dressing rooms, bathrooms, wc's or shower rooms) at floors above ground level are required to be suitable for escape in fire. In addition, rooms at ground floor level whose only escape route is via another room must be provided with suitable escape windows.
A suitable escape window is defined as 'a window whose unobstructed openable area is at least 0.33m2 and at least 450mm high and 450mm wide (a 450mm wide opening will need to be 735mm high). The bottom of the openable area should be no more than 1100mm above the floor. Any key required to open the window should be readily available.

K2 - protection for falling

Where a first floor window cill height is less than 800 mm above the floor level suitable guarding should be provided to prevent a person falling through an open window.
This requirement may conflict with Regulation B1 and provision of escape windows. One way of achieving the requirement may be to provide a restricted opening device that can be easily overridden in the event of an emergency.