cost effective lowering

Yes it would

If you want to lower your car the only way to go is a set of proper springs. Eibach would be a good place to start.

If you cut a spring sufficient to drop the car by say 15 - 20 mm how do you get an adequate spring seat?

All springs have the last coil manufactured to give a flat seat.  Cut that off and you don't have a spring seat.

If a broken spring is an MOT failure, surely cutting one is an invitation to a re-test?

If you cut then there is the problem that without doing the maths the stresses can be too much and the spring may not remain seated at full rebound.

In my opinion, and it's a minority opinion, these cars look good level. They are usually nose low and lower kits often make this worse.

If you cut, then a quarter coil off each end of the fronts is about the limit for the 316/318 spring because they are so skinny anyway. Don't do it if they have a lot of rust. This cut drops the front a few millimetres and gets a small increase in rate.

At the rear, more radical action is possible. I ran an E36 for years with 1.5 coils cut from the bottom end of the spring and the effect was great! This is only safe if the cut end is fitted downwards. You get m-tech ride height and a useful rate increase for 40 minutes work.