Better to ignore the shell case as if will make the question more complex ;-). It is a large navy shell case by the way, not a 7,7 cm, more specifically a 10,5 cm SK L/35-L/45 C/95. I write about this case in my "Krupp" article.
You have to agree the marking I show has a striking resemblance. Thanks for the link explaining this mark as a "sold" mark. I have seen it more often, but this mark is pretty rare.
That being said, the "sold" explanation would make more sense. It is very likely the British took a lot of WW1 stuff home for whatever reasons and sold it off later. I also have another strange image series of another large German case with a British primer and typical non-German/very-British slots at the mouth of the shell case. The British did do more weird stuff than documented for sure ;-).