Hi,
Welcome to the Forum.
Despite the 'Sheffield' mark, your teapot is American, made by the Poole Silver Co.
There are rival camps as to the attribution of some of the marks used by the Poole Silver Co., some think the Preisner Silver Co. were the user of some of the marks previously attributed to Poole, personally, I think your your teapot is the work of the Poole Silver Co. as Preisner were not founded until 1935 and the misleading mark 'Sheffield' stamped on non-Sheffield made goods was outlawed in the U.S.A. in the 1920's.
See:
http://www.925-1000.com/silverplate_P2.htmlTrev.