I like to clip my cables, always have, it's how I was taught when I served my apprenticeship. When dealing with lights in a loft, insulation isn't normally a big issue. An issue does arise however when dealing with bungalows. When you have a ring main and you're clipping it to the sides of joists and there's going to be, or is, more than 100mm of insulation, then you're going to be doing it in 4mm. 2.5mm gets derated to 17amps with this method and once a cable has been derated it has to be capable of carrying 20amps 433.1.5