The fascinating story of the interconnection between the UK mainland and the Isles of Scilly, off the most westerly point of the UK mainland, is told in historical documents of the South Western Historical Society (section Histelec section S48 article http://www.swehs.co.uk/archives/news48su.html).
The islands are connected to the mainland via a 33kV undersea cable, landing at Whitesands Bay (probably actually Gwenvor Bay, given the nearby connection of the underground cable to overhead line and the yellow diamond marker post above the beach).
The underground cable converts to overhead line at adjacent to Trevedra Farm, at the top of the hill above Gwenvor Bay.
From here, a 33kV line runs overhead and connects to the mainland network at St Buryan substation (arriving at the pair of poles at the farthest end of the lane).
This is the connection of the Isles of Scilly feed to the original St Buryan 33kV busbars which run left to right at the rear of the image below.
Viewed from above, the 33kV line is terminated on the two poles within the central grass rectangular area in the image below. It then runs underground for a short distance to clear the central white cabin, before exiting the ground and rising to the busbar level.
Due to the shallow waters around the Isles of Scilly, some of the undersea cable are visible.
Landing at St Martins can be seen snaking in from the bottom left of the photo.
The interconnection between the islands of Tresco and Bryher can be seen below.
The incoming 33kV cable from the mainland and the outgoing 11kV cable to St Agnus can be seen here at Porthcressa Beach.
Closer in, the two individual cable can be seen sharing the same route until the mainland cable separates to run the the east.
A video from Western Power Distribution with some interesting views of the power station on St Mary’s.