Here in the U.S., Mrs. C likes to add about two tablespoons or so of canned mlk to a stick (1/4 pound) of butter and whip away with electric mixer until achieving the desired consistency. It may take a little more or less to get the amount of whipping you desire. Keep in mind, this will not keep it from hardening once you put it back in the fridge, but is does seem to soften quicker when put out for use...
Here in the U.S., there's sweetened canned milk used for cooking, but this is condensed, evaporated, unsweetened canned milk, usaully marketed by Carnation Company, although there's others...