A long time ago I had a guided tour of Bentwaters (then an F4 base) and based alongside the Phantoms were SAR Hercs and Super Jolly helicopters. I was told that when AF1 crossed big water a series of Hercs would orbit on the line of the route (pronounced Rout) in case of trouble. Apparently they could have had PJs in the water within a relatively short time of AF1 getting its feet wet. The helicopters were fitted with refuelling probes, so their range was limited only by the crew's endurance.
It struck me at the time that only a very very rich nation could do that. At the time the Queen's flight had Andovers, I seem to recall.