Please bear in mind the AC system has benefits above and beyond just keeping the car cool during hot weather.
The system also dries the air in the car, eliminating condensation in cool damp weather as well as helping to keep the driver more alert in all weather conditions.
I'm not sure about the Up, but it is normal for car AC systems to shut down automatically when the outside temperature falls below 4 degrees centigrade. I always keep the AC system on all the time, partly due to the benefits above, but also because it helps to maintain the system in the best condition for the long-term. The refridgerant also lubricates the compressor, so when the system is on it keeps the system lubricated and helps stop corrosion in the compressor/system. AC systems can be expensive to repair, so a small increase in fuel consumption is likely to be cheaper in the long run, even without the benefits it offers year-round.
Whilst on the subject of AC, some manufacturers give the impression that system is sealed and needs no maintenance, but this is not really the case. Over time refrigerant leaks through the seals in the system at the rate of c.10-15% of total volume per year. It is therefore sensible to get the system serviced/re-charged every couple of years to maintain peak performance.
As this forum is read by a wide range of people, some with limited interest in technical issues, I hope the above is useful info. For those with more technical interest - it's not aimed at teaching you to suck eggs!