Unique ID

Is there a unique ID of the openhab instance that a binding has access to?

Yes, there is: https://www.eclipse.org/smarthome/documentation/javadoc/org/eclipse/smarthome/core/id/InstanceUUID.html#get--

and I think that you can also see it from the REST API: http://<OH2_IP>:8080/rest/uuid?api_key=UUID

Thanks - that’s exactly what I needed.