Is it safe to power the Uno with 18v?
It won't do any harm to try it at 18V for a while. Most voltage regulators have thermal protection and just cut out if they get a little too hot. If it works reliably, stay with it.

EDIT: Their Uno page warns against overheating the board, so I would test it for a short while and make sure there is no overheating or strong "plastic" smell coming from the board. If there's enough heat to worry, then it's best not to run like that for extended periods.

If 18V is too high, you could get a power resistor (I'd guess say a 10 watt resistor around 24 ohms) to waste approximately half an amp from the power supply. Connect this to the supply, in parallel with the Arduino.  It's wasteful (unless your room needs heating anyway) but will lower the voltage (that reaches the Arduino) a little.