It may seem odd to cook pork in milk, but the result is rich and tender. Funny how the British have a reputation for bland food. I'd say more like "subtle" or "un-fussy." Sometimes plain cooking is just plain good cooking.
Heat oil and butter in a large Dutch oven or pot that will fit the pork loin over medium heat; add the pork, onions, and garlic and brown meat well on all sides for about 15 minutes.
Add milk, juniper berries, and sage; increase heat and bring to a boil.
Reduce heat to low, cover and simmer until pork is tender 1 to 1 1/2 hours, turning several times to self-baste.
Transfer pork to a serving dish and keep warm.
Increase heat, if desired and cook liquid until it is a light brownish color (it will look curdled at this point).
To serve, slice pork in thick slices and season with salt and pepper, strain liquid then season with a little salt and pepper; drizzle a little of the strained sauce over pork, garnish with fresh sage leaves and serve additional sauce on the side.