4700 miles to St Helena as the crow flies. New York is 3439 miles away. According to
Saint Helena Island Info: All about St.*Helena ? Fly here? there will be a refueling stop in West Africa. The runway length appears to be 1550m, which is a tad over 5000ft.
Apparently Lubango Airport in Angola at a distance of 1,300 miles (2,100 km) is the closest diversionary airport for which every inbound aircraft must carry enough fuel reserve, limiting its load capacity as Ascension can't be used as it is a military airfield.
So what will the aircraft do when it is not flying to St Helena every two weeks? Will it live at Luton with its maintenance facilities picking up other work?