If Britain had a Thanksgiving Day what should it be thankful for ?

G.K Chesterton, who was Anglican for most of his life and converted to Catholicism toward the end, had this to day about American Thanksgiving (I think he can be forgiven for taking a little license with the facts)

In America, they have a holiday to celebrate the day the Pilgrim fathers arrived. In England, we should have a Holiday to celebrate the day they left.