Power on/off sensor (UK) (or inexpensive inline energy monitoring)

Suggestions please for a simple, small & cheap ST compatible sensor which can be poled to see if it has power (240v) applied?
I’m generally interested in small and cheap devices such as this rather than larger, expensive, multi-function ones for integration into 3rd party devices ad systems.
Thanks in advance

What country are you in? The device selection does vary.

Also, it would be easier on your network if you just got a notification when the power went out in that particular receptacle. Would that work for you? Many people have done this in the past, typically by having a plug-in device which will switch itself over to battery power when mains power is lost. You can get that notification without having to do constant polling.

Oh, yes, I’m new to this. I am in UK (240V).
Yes, receiving transitions of power on/off would be best, as you say.
Don’t really want the plug-in format, its for lighting. Guess I’m looking for devices which have / use a backup battery.
I’m looking to mirror the on/off state of a non smart circuit in one place with on/off in another.