I believe there is a reg that a habitable room must have an openable window area of at least 1/50 of the floor area. But that is a pretty tiny window even in a big room, so I would think your existing window should be fine on that front.
The Regs for a habitable basement room state that a window must be of a minimum unobstructed opening size of 850mm high and 500mm wide and should be between a minimum of 800mm and a maximum of 1100mm from the floor to the bottom of the opening and should lead to a place of safety free from the effects of fire. Where this is an enclosed back garden or yard it's length should be at least equivalent to the height of the building. You may also have to install Fire doors and such like. As GlynL suggests, contact your local Building Officer for a full breakdown.
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