Siren with Ring adapter

I’m wanting to hardwire the Ring siren, as don’t want to change battery every few weeks with position of the siren. So I have an old bell box, and so that existing wiring goes inside house where I could use the Ring adapter (I’m in UK), plugged in. This would mean I’d use 2 cores of existing wiring, and then join the Ring adapter ends to that cable. Has anyone done it this way? It’s impossible for me to use the Ring adapter wiring alone as too short and no wall socket near.