That's a cable coming out of a wifi repeater that someone has put in after market rather than the 2 prong plug which comes straight out of it when sold:
It's in a plastic box, so unless there's a ground contact on the PCB (which there won't be if it was designed for a 2 prong plug), there's no need to connect an earth.
If it came with a UK plug it'd be one of those with a plastic earth prong.
That's a cable coming out of a wifi repeater that someone has put in after market rather than the 2 prong plug which comes straight out of it when sold:
https://www.thingiverse.com/thing:6554999
Here it is unadulterated:
https://www.amazon.co.uk/AVM-FRITZ-Repeater-1200-AX/dp/B09N4SJLWM
Assuming I knew how to solder, would it be safe to wire it to a UK 3 pin cable/plug? Where would the third wire go, if anywhere?