There was a comment elsewhere that they need to be within range of land at all times in case a refuelling fails.

They are flying from: Patuxent River Maryland to: Fairford England which has a direct flight path of ~5800kms. The range of the F35 is ~2200km plus they need to maintain emergency fuel reserves and might fly a less direct path to stay near land. They are taking 2 kc130 air tankers for the trip.

http://aviationweek.com/farnborough-2014/priming-jsfs-transa...

http://www.aircalculator.com/flightplan.php?from=BZZ&from_ai...