It may not be working but I guarantee the sensor is there. It lives in the lower front bumper / spoiler panel with the fog-lights. The MFD (if fitted) or the normal dash-board display beeps and flashes when the outside temp falls below 2 deg C.
It would worry me if someone has gone to the trouble of disabling all of this hard-ware and software functionality. What else have they done?
Maybe you need to cycle or scroll through the various displays for the trip computer using the button on the end of the wash/wipe stalk (R/H side). This displays current fuel consumption, cumulative fuel consumption, ambient temp, trip mileage, remaining fuel distance, etc, etc.
Yeah, I was surpised as my 05 Golf has the on-board computer. The A3 only has little stick things beside the odometer which you can press to display the kms to next service etc. As the car has a/c I was convinced that it would display at least the outside temp.
Looks like a case of an "Irish spec" car as you say.