B&Q currently have an offer on all smoke alarms 3 for 2. Even at £72 each the units are not cheap but offer combined smoke alarm and carbon monoxide sensor.
I haven't seen these units discounted anywhere. This is only available in store. I purchased 6 in total, 3 from the Kidderminster store and 3 from the Wednesbury store
Nest Wave- No more swinging brooms or towels. Silence alerts with a wave of your hand.
Heads-Up- Get a friendly alert so you can handle burning toast before a piercing alarm goes off.
Know from anywhere - Sends a message to your phone if the alarm goes off or if batteries run low.
Nightly Promise - Nest Protect tests its sensors and batteries and lets you know it's working with a quick green glow at night.
Silence the chirp - Low-battery chirps ever rattle your dreams? Nest Protect shows you its batteries are good. Before you doze off.