Trim the pork loins, exposing about 4 cm of bone at the top (or ask your butcher to do this). Remove the skin and score the fat in a diamond pattern. Mix the juniper berries, peppercorns, herbs and grated rinds and rub the mixture over the fat. Put the meat, fat side up, into a shallow dish and add the gin. Cover the dish with foil and refrigerate overnight.
Preheat the oven to 190°C. Stand the loins upright, with the fat outside and bones interlocking, in a roasting pan. Tie the joints securely with string between each pair of bones. Put the roasting pan in the oven and roast the pork for 2 1/2 hours, basting occasionally. If the meat browns too quickly, cover it with foil. The pork is done if the juices run clear when a skewer is inserted into the centre of the joint. Transfer the cooked pork to a heated dish, cover and leave for 30 minutes.
Meanwhile, to make the sauce, bring the milk to the boil in a saucepan with the bouquet garni. Cover and set aside for 1 hour, then remove the bouquet garni. Melt the butter in a saucepan, stir in the flour and cook over medium heat for 1 minute. Gradually stir in the milk and bring to the boil, stirring, until thickened and smooth. Simmer for 2–3 minutes. Stir in the mustard and the cream, season to taste and continue to cook on low heat for 3–4 minutes. Pour into a heated sauceboat and keep warm.
Remove the string from the pork guard of honour and garnish with slices of lemon and lime and the parsley sprigs. Carve by cutting downwards between the bones.