This is a National Trust property situated in the middle of Poole Harbour in Dorset, a few miles north of Swanage where I have based most of my Dorset holidays. There has been a settlement on the island since 500BC, and after becoming the property of the Crown in 1530s, a fortress was built on the island to defend Poole Harbour. The mid-1800s saw the island become part of a pottery and clay industry, although the clay turned out not be as good quality as initially thought, and the industry ceased in 1887, the island reverting to agricultural use. This was abandoned in 1927, and the National Trust took control of the island in 1961.
Getting to the island from Swanage requires two ferry trips (unless you go all the way round Poole Harbour to Sandbanks). However, there is a National Trust Car Park next to the chain ferry toll booths on the Studland side of the harbour entrance, and this is free to NT members. From this, you can use the chain ferry to access Sandbanks and then use the ferry service from there to the island.