Part of the problem IMHO is the inadequate wiring space in accessories and back boxes. In many cases the larger loading electric showers will need 10mm cable. It is almost impossible to fit two lengths of 10mm twin with earth correctly into an average pull-cord shower switch. Similar arguments apply to cooker control units.

Even socket outlets and standard back boxes do not readily accept 3 twin with earth cables each of 2.5mm.

A great many real world installations have the outer sheath of the cable terminating outside of the back box, for lack of space to do otherwise.

USA rules are superior in this respect, requiring a minimum wiring space in cubic inches. 

Part of the problem IMHO is the inadequate wiring space in accessories and back boxes. In many cases the larger loading electric showers will need 10mm cable. It is almost impossible to fit two lengths of 10mm twin with earth correctly into an average pull-cord shower switch. Similar arguments apply to cooker control units.

Even socket outlets and standard back boxes do not readily accept 3 twin with earth cables each of 2.5mm.

A great many real world installations have the outer sheath of the cable terminating outside of the back box, for lack of space to do otherwise.

USA rules are superior in this respect, requiring a minimum wiring space in cubic inches.