This demo will only work if a) you are actually allowed acces to the underlying GBN route data, and b) you've already authenticated by Raven to annotate.map.cam.ac.uk. The easy way to test the former and achieve the latter is to try to view the UcanGeoJSON representation of the GBN. Note that this demo is quite slow since the GBN data taxes the overlay system.
Assuming your have the necessary authorisation (and are authenticated to annotate.map) you can also view the GBN overlayed on the main map at https://map.cam.ac.uk/#https://annotate.map.cam.ac.uk/adapters/gbn.json.