If you have an airlock the heater may not work very well and / or the engine may overheat.
The radiator can be drained by releasing the bottom hose connection. The cylinder block drain plug is on the rear face above the starter motor.
Bleed screws are located in the radiator top hose and in the coolant pipe on the front corner of the cylinder head. MINI One /Cooper have an extra bleed screw located in the heater hoses beneath the battery tray.
On refilling slacken the bleed screws, set heater control to max. temperature, fan speed low, which opens all the heater valves. Fill via the filler pressure cap neck closing bleed screws as bubble free coolant emerges. Now fill the expansion tank to MAX mark. Start engine and run at idle speed, until coolant level in the pressure cap neck ceases to drop. Turn off and top up to the top of the neck again. Restart, run up to temp then allow to cool completely and recheck all levels, repeat checks as necessary and watch out for any signs of overheating for a few miles.
(The Cooper S has the pressure cap located on the expansion tank so has a slightly different filling method. to One and Cooper)