Without being able to ask author John Wyndham, I can only give you a few ideas that I have.  I would also like to point out that the story starts with a dream/vision of the Sealand city, but the story ends with David actually seeing the city.  I have three possible reasons for why the story might begin and end this way.  

1. By beginning and ending the story with the Sealand city, the story is nicely held together by a set of "bookends."  The ending reminds the reader of how the story began, and it gives readers a sense that the story has ended with David "returning" home.  The beginning and the end sandwich the middle.  

2.  David's dream/vision of the future fits with David's character.  He is a telepath throughout the story, but that doesn't mean it is his only special mental ability.  His powers emerge and grow through the story, so it is possible that glimpses into the future are also a part of his abilities.  David's vision at the beginning of the story might be a sign to readers that David has or will eventually have a "glimpse" ability. 

3.  The opening dream/vision is the author's way of foreshadowing the Sealand people and their great city to readers.  The book has some tense moments, but deep down readers know that David's dream is likely to come true, because the opening paragraphs of the book point toward that outcome.