Here is a really neat product:-
“Never worry about changing batteries again. This unique smoke detector screws into any ceiling light fixture and operates off your home electricity. Reinstall the light bulb into the alarm, and now you’ve got a light source and smoke alarm in one! Recharges itself every time you turn the light on.”
Be cool to make one that works with a standard UK ceiling rose and even better that uses the house wiring to activate all the alarms in the house.
Maybe one exists?
Thanks to the Make blog for the link.