1. Line a 9-by-4 1/2-by-3-inch loaf pan with parchment paper, allowing at least 3 inches of overhang on the long sides. In a small bowl, sprinkle the gelatin over the water and let stand.
2. In a medium bowl, using an electric mixer, beat the egg whites at high speed until firm peaks form. In another bowl, beat the cream until firmly whipped; refrigerate.
3. In a large bowl set over a pot of simmering water, beat the egg yolks with the honey at medium speed until thickened and an instant-read thermometer inserted in the yolk mixture registers 160°, about 6 minutes. Remove the bowl from the pot and beat in the gelatin mixture, lemon juice and zest; continue beating until slightly cooled, about 5 minutes longer.
4. Using a rubber spatula, fold the beaten egg whites and whipped cream into the lemon-honey mixture until no streaks remain. Pour half of the semifreddo base into the prepared loaf pan. Arrange the ladyfinger sandwiches in the pan in 2 long rows and top with the remaining semifreddo base. Cover loosely with the overhanging parchment and freeze until firm, at least 4 hours or overnight. Unmold the semifreddo and peel off the paper; slice and serve.