Britain will have four of its naval aviators serve on American aircraft carriers over the next decade, to maintain Royal Navy knowledge of how pilots operate jet aircraft off carriers. The British naval officers will learn to fly F-18s in order to do this. While Britain and the U.S. regularly exchange fighter pilots, this is a special case.
That's because, earlier this year, the Royal Navy retired all its Harrier aircraft and the last aircraft carrier that the Harriers operated from. That presented a problem, as the first of two new carriers won't enter service for a decade. The admirals knew that once the new carrier (Queen Elizabeth) entered service, a new generation of pilots would have to be trained to take off and land on a carrier. While the Harriers could land and take off like a helicopter, they often took off (via a "ski jump" flight deck) so they could carry more weight (especially bombs) into action. The new carriers were originally going to have the F-35B which, like the Harrier, can take off like a helicopter, or by rolling down a carrier flight deck. But that has been changed, for economic reasons, to the F-35C, which operates like the F-18E (no helicopter type operations). The Queen Elizabeths will have a catapult, like current American carriers do (and pre "ski jump" British carriers did as well).
The current steam catapult was designed by a British naval officer in the 1950s, and were essential for launching the heavier jet aircraft. The British eventually abandoned the large carriers, and steam catapults, because of cost. But the new Queen Elizabeth class carriers are 58,000 ton ships, and a return to the use of big carriers equipped with steam catapults. Each of these ships will carry 34-45 aircraft and helicopters and be able to handle about 110 flight operations every 24 hours (mainly with F-35C). The British know from experience that it's easier to train new pilots with experienced Royal Navy carrier pilots. Thus the need to maintain that experience by having British aviators flying F-18s off American carriers until the new British carriers arrive.