Suspension springs have flat tops and bottoms, to have a secure / strong fitting. By cutting them, your putting all the stress onto a single point instead of the base of the spring
I wouldn't be surprised if this caused the spring to be less effective and a higher chance of snapping under load.
Do the job properley, and get lowering springs. It's just dangerous cutting them to save a few quid. If you lose control of your car as a result, you could end up killing someone! If they weren't dangerous - they'd pass MOT's wouldn't they!