Pour boiling water over the poppy seeds and leave for 1 hour to soak. Meanwhile, heat the milk in a small saucepan with the butter until the butter has melted, then leave to cool until lukewarm.
Combine the flour, yeast, sugar and salt in a large bowl (making sure not to pour the salt directly on to the yeast, as this can deactivate it). Add the milk, butter, egg yolks and vanilla paste and mix until well combined. Lightly knead for 10 minutes on a lightly floured surface until the dough is springy, then cover with cling film and set aside in a bowl in a warm area to rise for 2 hours, until doubled in size.
Place the walnuts and raisins in a mini food processor and blitz until finely chopped. Drain the poppy seeds, place in a food processor and blitz for 3-4 minutes, until mostly broken up. Add the walnuts, raisins, sugar, honey and cinnamon and blitz again until finely chopped. Whisk one of the egg whites with a handheld electric mixer until it forms stiff peaks and stir it through the poppy seed mixture with 2 tbsp chopped mixed peel.
Preheat the oven to 190°C/fan 170°C/gas mark 5. Once the dough has risen, roll out to a 28 x 23cm rectangle about 5mm thick. Spread the filling out over it, leaving a 2cm edge. Starting at a long end, roll the dough up in a spiral, making sure that it's not too tight. Turn the ends under the roll so that the filling doesn’t leak out. Cover with cling film and leave to prove for 30 minutes, so that it's slightly puffed up.
Brush with another egg white and bake for 20 minutes until golden all over, then cover with foil and cook for a further 10 minutes to make sure that it is cooked through. Leave to cool. Once cooled, mix the icing sugar with the lemon juice and drizzle over the cooled loaf. Sprinkle over the extra poppy seeds and mixed peel and leave to set for 10 minutes before slicing and serving.