Turning a circuit off using a 240v signal
This all sounds far too complicated. You need something simpler, like a thermal fuse. This is mounted on your hot tank, and goes in series with either your heating PID controller, or the kettle element.
In a few seconds with google and 'thermal fuse' I found a supplier of 73°C, 15 A, 240 VAC fuses for less than 1$ each.