What would be the best way to power 10 AAA powered torches with mains power (240v)
This depends on the design of your torches (CanEnglish "flashlights).

Many small LED flashlights powered by three AAA cells depend on the internal resistance of the AAA cells to limit the current through the LEDs.  If your flashlights are like this, with no separate current regulation, then powering them from a normal DC power supply will likely burn out or damage the LEDs, as the power supply would probably be capable of delivering more current than the LEDs can tolerate.  In this case, you would need a current-limiting regulator for each flashlight you wish to test.

If your flashlights do have a current regulator, then you could safely use a normal DC power supply of the appropriate voltage.