I have a toyota with the same engine - this 1200rpm idling sounds normal - when others have said air con i think they mean climate control.
Having the heaters on 21 degress or above with an ambient temp of 0 degrees or lower the engine will idle at 1200rpm. As above but ambient temp between 0 and 10 degrees it will idle at 1000rpm.
The engine will idle like this until the engine has reached FULL operating temp.
The climate control (well heaters) utilise a PTC heater for UK spec cars - this requires (in toyota vehicles - i imagine lexus will be the same) 3 x 30amp relays as they draw a lot of electric!
I assume this is why the engine increases it's idle when these PTC heaters are active.