Just to get it started, your object lacks a full set of marks to be able to call it "hallmarked". One possibility is a pseudo sovreign head mark used outside of British controls. Using the search box, there are many references on pseudo head..I don't want to point to any particular origin, but here is one thread that helps to understand the use of such marks including the sovreign head in oval panel or cartouche:
viewtopic.php?f=3&t=55076&p=175897&hili ... 3d#p175897
Can you find any other marks like a letter? Look in the British Hallmarks section (scroll down) and you will see five heads..the Victoria with plunging neckline might also be an influence for a (pseudo) mark like you show in the images. Now we wait to see if someone can find more and better info for you.