The Tom Tom Go is very good. The mount has the power connection, so if you clip it on the mount it start charging.
Get the car mount from www.brodit.se
for it. You need two bits from brodit. One fits the MINI, one fits the Tom Tom Go, and they are screwed together.
The whole lot is a fraction of the price of the stuff from BMW, can be fitted in minutes, can be used on its own without a car (which is nice for planning a route at home), and is plenty loud enough to be heard when driving.