how does the narrator describe the magna charta island ??

From the text:

"We had originally intended to go on to Magna Charta Island, a sweetly pretty part of the river, where it winds through a soft, green valley, and to camp in one of the many picturesque inlets to be found round that tiny shore."

when the charter of England's freedom got signed by King John , his mercenaries and warships left the shores of that island and the island got a new name "Magna charta island".