Carrack Looz en Cooz, better known in English as St Michael's Mount, is a castle-church-manor on an island in Britain's most westerly sheltered bay, appropriately enough called Mount's Bay. Owned by the St Aubyn family but worked by the National Trust for cultural and preservation purposes, the Mount is open to visitors for most of the year, although opening days are restricted during the winter.
That said, it was perfectly nice in February (if unsurprisingly grey), and the nice thing about going in the off-season is that it's not crammed with people and you have a local guide shepherding you about. I mean very local -- lives in one of the houses by the harbour you can see in the picture sort of local.
This view takes you over to the peak Trencrom and the hills of Penwith, the most southwesterly district in Britain. (Interestingly, as you can see from any map of the UK with longitude lines, there is a fair deal of Scotland that is further west.)
At low tide, you can walk over the causeway, but at high tide, you end up taking an amphibious boat-truck thing to the harbour. I totally want one of my own.