There was a real issue this time Jack, though I accept that things are sometimes cancelled when there seems little reason.
The reason wasn't the open sea state as such, though at the time the wind was gusting >50mph (even on the land) but the wind direction.
All major ports in north and west Cornwall are sheltered from the W & SW for a very good reason (Padstow, Newquay, St Ives, Penzance/Newlyn, Falmouth etc.) A problem arises during an E gale, which this was just offshore. The SCIII is flat bottomed because of the shallow conditions, both at Penzance & Scilly, which makes it roll & makes docking tricky in rough conditions. With the wind gusting to around 50 mph on the lighthouse pier Penzance (46mph at my sheltered site set well back from the sea front), and the waves coming over the pier, the sailing could not go ahead.
The prom was awash. Even if the ship had docked, the passengers would have been bombarded by pebbles and weed flying over the pier.
I used to work for the Steamship Co., who run both the Scillonian & Skybus flights from Lands End, Newquay & Exeter. There were many occasions when the sailing went ahead and ambulances had to meet the ship due to minor injuries & severe sea sickness. It's complete open Atlantic off Lands End, many (not all!) Scottish ferries are between Islands, so there is some shelter.
Graham
Penzance