Countdown - Nintendo DS Gameplay (2009) 

A look at the DS 'Countdown: The Game' by MindScape and released in late 2009. A Nintendo Wii version was also released.
As well as the single player mode, there's also 'Training Mode', whereby you can play an endless series of any of the three Countdown disciplines, as well as multiplayer mode over the DS WiFi - sadly, this doesn't work in the form of playing a regular game, instead playing a 'best of 3' concept in either letters, numbers or conundrum, unfortunately.
I'd say the DS version is probably the best official Countdown game you're gonna find anywhere (although that's not saying much, to be fair).