Saw these in a POS display in Swindon Tesco. Looks like a promotion from Energizer rather than a Tesco thing so I'd assume it's nationwide and maybe not even exclusive to Tesco.
I'm sure there are cheaper mail/Internet order places, but this seems like a decent enough deal for those who don't like waiting for things, especially if as in my case your Wii-mote needs juice.
Speaking of the Wii, I got my first perfect game in Wii bowling yesterday. Only took 190 attempts!