Cat β Mike was correct, but the attribution is much more interesting than just a department store.
You are fortunate in owning what I believe to be the most romantic English glass collectable. The mark is actually an "S" over a Plimsoll Line, painted on the side of ships to ensure that the ship is properly laden. The "S" was the mark of John Stonier & Co. of Liverpool, principal wholesalers to the shipping lines, including the White Star line, and is found on glass and china, and possibly on other goods. I believe that Stoniers was also a Liverpool department store.
All the examples I have seen have been on Stuart uncoloured cut lead crystal, but you cannot assume this, as Stoniers could have sourced from other glassworks. There are at least two versions of the mark, one with a thick curly "S" and another with a thinner, more angular "S" with hints of corners. I think the second of these could be the later. Obviously, as it is acid-etched, the mark was applied at the glassworks.
While we know the mark was used on stemware and tableware used on board, we don't really know whether it was used on glass souvenirs sold on board, but I believe this also to be true.
So, what you have is glass either kept or bought on board an ocean liner as a souvenir of a voyage, or as a gift.
It is the easiest glass in the world to sell with all the romantic connotations, but not easy to find. I've not found any for over a year.
While I have no objection to anyone using this information, acknowledgement would be appreciated.