My references show the silverplate marks starting with a fluer-de-lys and gondola mark were used by Deykin & Sons (D&S), Birmingham, England. This gondola mark was also used on their nickel silver pieces trademarked "Venetian Silver".
In 1895 the name of the firm changed to Deykin & Harrison (D&H).
Their sterling mark changed from a gothic style to a standerd print around 1924. Seems reasonable to me they would have changed the silverplate marks around the same time. I know they changed it because I've seen both the Gothic and standard print used with this silverplate mark. So, I'd say your teapot probably dates to the first quarter of the 20th century.

Regards,
Pat.