This is instore only. Nearly all of these are reduced to half price, I have looked online and they are still full price. Please note that the store I went into had far more of the range on sale than what is online.
I just bought a basic Carbon Monoxide alarm CO-9B, reduced from £20 to £10.
If you want something more elaborate then the Premium WiFi Smoke and Carbon Monoxide kit is reduced to £31.81 from £63.62 http://direct.tesco.com/q/R.204-9728.aspx
With lots of others in between those prices.