We put a bottle in the fridge with cooled boiled water in it, and leave about 50-70 mls free and then we just boil the kettle. If we boil it before we go to bed, it doesn't take that long to boil again (mind you, we go to bed at like 11.30 or something!) so she's only really waiting about a minute. Then we put in the formula and make it up fresh.
That is exactly what I do and it works a treat. I have such a small amount of water in the kettle too, just enough to cover what i need to use, and to not burn the kettle out. I have the formula measured out and ready to go, as my DS only really need a bottle maybe once a week in the night now, so this is the easiest way for us.