The European cranberry, Oxycoccus, pink flowers and red berries, grows in Britain. Another related plant, but usually called "cowberry," also grows in Great Britain.
Then there is the bush cranberry, Viburnum opulus, which grows both in Europe and North America.

The bog cranberry, Oxycoccus macrocarpus, which shows up with turkey at Thanksgiving, is North American, but there is no reason to equate the Cranberry Isles of the song to this plant.