If the temp is constant below red range in the gauge, it's god to go. Slightly above half is the normal range in many Minis. However, if the car is not boiling, nor you feel very hot inside the passenger area, behind the firewall it's good to go. I hope you run with a 3 core radiator. If your engine is for leaded fuel and if you run unleaded, there could be a working temperature increase.
All in all, still there is a fault in your heat exchange system as you experience 2 different working temperatures with and without A/C - when you use a thermostat, working temperature should be constant.
If you have 45Amps alternator, you will experience battery drains if you run short distances with head lights, wipers, A/C, stereo, and fog laps on. It's better to have a 75/90 amp new alternator - you can buy an Indian one for around Rs.3000- or can get a brand new Lucas one for around Rs.6,500-. the same alternator goes in to Land Rover, Massey Ferguson tractor and for some other English cars.
Japanese used alternator is the best option (as charging is controlled by ICs) if you do not worry on using English or English look alike spares.
GMI