We are given the answer to this question in Chapter 8. Actually, it is Bernard who makes the decision to take John and Linda back to the "Brave New World" where they came from. Unfortunately, however, it is made self-evident that this decision is not out of generosity or any kindness on Bernard's part:
"I wonder if you'd like to come back to London with us?" he asked, making the first move in a campaign whose strategy he had been secretly elaborating ever since, in the little house, he had realised who the 'father' of this young savage must be. "Would you like that?"
It is made clear, then, that inviting John and Linda back is a self-serving move on the part of Bernard. He hopes to use them against the Director of the Hatcheries, who is threatening Bernard and his position. Even though the the idea of Linda disgusts Bernard, he realises that "her very revoltingness might prove an enormous asset." Indeed he is correct, as the "unveiling" of the savage and of Linda causes the immediate retirement of the Director, but unfortunately, John does not prove to be as pliable as Bernard would like in the later chapters.