It was originally set up to be a simple process: That hasn't changed despite the proliferation of different, overlapping bodies. If it's an adult ... contact the local Police.
If you think it's a child, ....contact the local Police.
Yes, you can go to the Local Authority, but they are gonna contact the Police immediately as it's a Police issue primarily. Yes, the LA will become involved to an extent, but the Police are your first point of call