It's normal for engine to load up when load (lights) is applied to alternator. As for how much it drops, depends on lots of factors, engine condition/tune, amount of load i.e. high wattage lights etc. I'm assuming it's just a normal Aussie mini, not injected Rover one. A quick test would be to unplug each headlight, preferably high beam, start engine and then whilst idling, switch light switch on. Obviously only parklights will come on but there shouldn't be any significant RPM drop. If there still is it will require more testing. Try it and let us know outcome.
Not trying to be a smartie or anything, but I'm assuming you're not a mechanic or the like. The alternator would normally run everything and have enough reserve to put some back into battery. Basicly for it work, it needs to 'make' power, to do this it draws power (HP) from the engine, the more power you want from it, the more it draws from the engine. .................Basicly
