All the circuits, including the lighting, in my house run through a leakage breaker in the fuse box. Damned annoying if it tripped after dark, until I installed a battery emergency lighting system.
That is precicely the reason that lighting circuits should be left off the circuit breaker , as should freezers and grannies iron lung be fed seperatley.
Domesticly the breaker should only be on the downstairs ring or any where there is an external power supply, also the immersion heater which should have a dedicated feed.The parasitic supply mentioned, this is the way lighting dimmers are powered, i have seen a few melted cases on those.
Its a bad idea, could cause a fire and would give a real insurance problem if a fire occours and its not an approved device.In the uk typically the switch is purely a dropped t and e switch pair so when the switch is on both red and black are live.
If a neutral is present its likely to be straight through with no slack to cut and get a connection.