Question:
Can anyone see a reason why I shouldn't put a large contactor on my consumer unit that cuts off every circuit except the lighting and fire alarm circuits when the fire alarm is activated? Waste of money? Sensible precaution? How often are house fires exacerbated by energised electrical systems?

Answer: I looked up the figures for deaths in the home (which could be different ration from numbers of fires) some time back and don't recall precisely, but I think deaths due to electrical fires as opposed to deaths due to non-electrical fires was not a large proportion. Smoking and chip pans as causes of fires in homes were way higher. In the case of a fire, your lighting circuit might have already failed, and you might be relying on a bedside light plugged into the ring circuit initially. My HA system actually switches all the lights on if the fire alarm goes off.