I've never heard that cutting a spring makes it weaker. A shortened spring is stiffer - takes a greater force to compress it by the same amount. Usually measured in ilbs per inch if you are my age! It will obviously be shorter (that's the whole point) and you risk it falling out of the spring pan on full droop. But that is only likley if you get 4 wheels off. The fronts can't fall out as the piston of the strut goes through the middle, but it can come out of the seat. I suppose the rears could come out completley. But they didn't on the last rally I did which had numerous high speed yumps (Epynt). I have never had a problem getting a car MOT'd wth all sorts of modifications, providing they are done properly.
Don't forgetto getthe tracking checked after you lower it. Both the camber and tracking will be affected on the front by lowering the car and you may get greater tyre wear on the inside edge of the tyres because of the camber change.