Hello hilg,
There was a German community in London, called “Little Germany" by the English, concentrating in the quarters Whitechapel, St. George’s and Mile End in the East End. There it was specific streets in which the German immigrants preferably settled (Lit.: Manz, Stefan: Migranten und Internierte: Deutsche in Glasgow, 1864-1918 ). Most of the Germans were members of the German Church in Whitchapel. Perhaps your predecessors can be found in the church archive.
Another possibility: Try the Anglo German Family History Society that has regular meetings at King’s Cross. The East London History Society might also be of interest for you. There are two very good historians who are doing historical research and family research. Their names are Derek Morris and Ken Cozens. As soon as you found out the birthplaces in Germany, I could send you a few hints where to get further information on the German side.
If you have any photos of the shop front, showing the German specialities on display in the window, I would be interested in getting a copy.
Regards
Histres