Not all of the hedgehogs used for this movie are the species that live in the wild in England, which have brown underbelly fur. Some are a larger specimen of the African Pygmy variety of hedgehog that are easily found at pet stores in the United States. These are the ones shown with the white faces and bellies and "chocolate chip" colored spines.
This film has been criticised for the fact that Mary and Colin have ended up marrying each other despite originally being cousins in the book. However, this is considered a sequel to the 1987 adaptation and they weren't related in that.