Fans of the late Terry Pratchett will recognise the line-of-sight telegraph system as the inspiration for the Discworld's "Clacks" network.
I have stayed at two Landmark Trust properties with operational flagpoles, and one where you can flag down passing trains at its own private halt, but to have one with working telegraph semaphore arms would be even more interesting, (even if the adjacent links in the relay have both vanished)