Above is a set of unknown Hanau marks, see: https://www.925-1000.com/Fgerman_hanau_marks_01.html
However, a clue to their possible origin may be found in the details of the English importer.
R.G.A. - London - 1898
Rosina Gertrude Alexander of R.G. Birch was declared bankrupt in 1900. Among her creditors were the Hanau manufacturers, Schleissner Söhne and Weinrank & Schmidt. Although nowhere near conclusive, this information may offer just a clink of light as to the possibility of origin.