Question:
If a battery only smoke detector senses low power it chirps for help. Why doesn't a mains smoke detector have a battery or other storage device to drive an alarm to warn of lost power? Or even to provide back up smoke detection if power fails? Burglar alarms have this kind of backup.

Answer: -They do have battery backup. You can get types that don't, but they are rare, not recommended, and require special installation techniques, like having their own way on the consumer unit.

-"It is safer to connect to a light circuit than have a dedicated one. This is because the MCB tripping is more likely to be noticed as the lights would stop working. A dedicated smoke detector circuit going down is unlikely to be detected for weeks."

but if the smoke detector circuit has battery backup and chirps of it loses mains power then how can it go undetected for weeks?

Unless it only chirps when the backup battery is going flat.

In which case modify my query to "Why doesn't it chirp straight away? Or at least after 30 minutes or so to cover temporary off/on scenarios?"