I have come across an entirely different kind of explanation for Sir Thomas's trip to Antigua than the one that is suggests it is a way of making the issue of slavery an important piece of the background context of Mansfield Park. In one of her series of essays on Austen in Scrutiny X (19421-2), 137ff., Q.D. Leavis suggests that Austen may have gotten this idea (as she did others for other of her novels) from Elizabeth Inchbald's A Simple Story.
The argument runs: in Volume 3 of A Simple Story Lord Elmwood has to leave his wife "'in order to rescue from the depredation of his steward, his very large estates in West Indies. His voyage was tedious; his residence there, from various accidents, prolonged form time to time' [quoted by QD from Inchbald]. In consequence, Lady Elmwood became dissipated" (pp 137-8). In other words in both novels both paternal figures go off for an unexpectedly prolonged journey very far away, a trip which shows the paternal figure at least to be hard at work (never mind what the nature of the work is) and just as the Bertram family loses its moral bearing when Papa is away so does Lady Elmwood.
Perhaps Austen has Sir Thomas go to Antigua because she half-remembered the device from A Simple Story as one element in a complex of motives and knowledge about slavery and sugar prices that led her to send Sir Thomas away with a given year in mind (though which exactly is admittedly hard to say). Or maybe she just used Antigua to give her narrative or device remembered in the earlier book that believability things are given by given them probable detailed circumstances which fits in with what people have been reading in the newspapers lately.
I remember not too long ago someone on this list said she had just finished reading Inchbald's A Simple Story and said it was very good. If this person reads this posting, I wonder if he or she could comment on this parallel between Inchbald's and Austen's novels.