X2 capacitor voltage?
These are X2, hence across the line application rated capacitors. By definition they can handle power lines. In this case 275VAC. So also 240AC.

Peak voltage is indeed sqrt(2) times higher but these capacitors are even meant for much larger peak surges.

It's all in the datasheet, really. Are you sure you know what you're doing? If you need these then you're dealing with power line applications. This could be dangerous without the proper knowledge.