Welcome to OpenStreetMap and thanks for adding this.

Rather than adding the tag to Finsbury Street, this should really be mapped as a node (point) on the outline of the 20 Ropemaker Street building. You can connect it to Finsbury Street by adding a short length of service road.

There is a link to the wiki page for amenity=loading_dock below, but if you would like any help please feel free to ask.

https://wiki.openstreetmap.org/wiki/Tag:amenity=loading%20dock