Lowest power circuit to control low current 240v circuit
Some solutions...
Opto-triac with low LED current. This should not require a special snubber as, during turn-off, the triac will stay on until solenoid current reaches zero.
FET Solid state relay with low LED current. Being FET based, this will need a snubber to dissipate the inductive energy in the solenoid. Or you could use this kind of component to drive a snubberless triac.
These have less than 5mA input current so can be driven by a GPIO.