Franz Wilhelm Dresen workingin Christiana in 1870s

My grandfather, Franz Wilhelm Dresen (aka William Dresen), born 1850 in Elberfeld, Germany, lived in Christiana after 1874 for some years but moved on to London, England where he appears in the 1881 Census. Can anyone suggest ways of finding out what he was doing and where while in Christiana? Did foreigners have to register with the police and if so would their records be available? The last entry in his Prussian passport is for December 1873 when he took a job in Nurmburg, Germany. The British 1881 Census gives his occupation as Tin Worker; certainly he had expertise in making metal boxes and in tin canning (of food). (Unfortunately, about 50 years ago my grandmother destroyed a large quantity of letters from a lady in Christiana to him.)