If you have the On Board Computer, you can "clear it" while driving a steady speed and you'll get an instantaneous average speed that will be your actual speed.
You should also know that the MINI's speedometer will be slightly off by design (but certainly not by the amount you think yours might be misreading).
I posted the info below previously, but it applies in this discussion:
An interesting article in the the April 2002 issue of Car and Driver concerning speedometer accuracy explains why European cars have speedos that read slightly low.
The article asks the question why European cars typically have speedometers that do not reflect actual vehicle speed speed as accurately as speedometers in cars from North America or Asia manufacturers (based upon years of Car and Driver test data).
The article specifically mentioned BMW and Porsche speedometers (which seems to include the MINI). Of course having a European car with a less accurate speedometer is counter-intuitive when you consider the high speed autobahn travel possible in parts of Europe.
The upshot: A certain level of inaccuracy is designed in on purpose by the manufacturer in order to meet EU regulation ECE-R39. Mostly this has to do with making sure the speedometer never underreads true speed. This is important, the EU feels, because a car owner may fit a variety of different -- usually larger -- wheel and tire combinations to a car.
Here's the real interesting tidbit: Trip Computers do not have to meet this EU regulation, so they tend to read more accurately. Meaning if your speedometer says you are going 80 mph and you maintain that speed and reset your trip computer to determine your real average speed, the Trip Computer will show the more accurate, (i.e. slightly lower), true vehicle speed. I know that on my Cooper S, 70 mph on the speedometer reads as 67.8 mph on the computer.