As a great novel, this text has many examples of foreshadowing, however you have highlighted an excellent example in the dream that David has, which foreshadows the ending of the novel and his arrival at Sealand. Another example that is very poignant comes in Chapter 7. David's mother's sister, Aunt Harriet, comes to visit with her new child, just as David's mother has just had Petra. However, it is clear that unfortunately this child, like the two others she has had, is classed as a "deviant". Harriet makes it clear what kind of position this leaves her in:
"This is the third time. They'll take my baby away again like they took the others. I can't stand that - not again. Henry will turn me out, I think. He'll find another wife, who can give him proper children. There'll be nothing - nothing in the world for me - nothing. I came here hoping against hope for sympathy and help. Emily is the only person who can help me. I - I can see now how foolish I was to hope at all..."
What this event foreshadows is the hardness of David's father and his mother in their harsh obedience to the purity laws - even at the cost of turning away family to an uncertain fate. Just as they turned away and rejected Harriet, so too will they turn away and reject their own son and daughter. We learn at the end of the chapter that Harriet killed herself, and presumably the baby, out of despair. This of course paints a terrible future for anyone who is either deviant or trying to hide a deviant individual.
If you want more examples of foreshadowing, you might want to consider David's dream about his father sacrificing Sophie as he sacrifices beasts. Hope this helps! Good luck.