Mainly impacts on oil quality since the engine and oil never reach full operating temperature. The result is that the moisture that collects in the oil never burns off and oil quality (and therefore ability to trap contaminants) suffers.
I have a 2009 Cooper D and do 10km to work in the morning and 10km return to home on a daily basis. I usually take it on a highway run during the weekend (75km minimum) and have done so for the last 5 years. I have never had any engine issues, but I do follow the harsh duty cycle schedule for engine oil changes (meaning more frequent changes than the onboard computer suggests). In my case, I do the oil roughly every 10,000km.
Currently I am almost at 125,000km and the car is doing well!
Not sure if the SD model has a particulate filter or not, but on the 2009 diesel engine, to burn off the accumulated soot the car needs to be run at more that 60km/hr for about 15 minutes which usually is only possible on a highway trip. In addition, an engine not at operating temperature will not burn fuel efficiently and therefore will cost you more money in fuel.