Williams and Powell, of Liverpool were very respected makers and produced some very high grade guns, many of which were destined for America. They were successors to Ann Patrick who built fine flintlock guns in the late 18th and early 19th century. Liverpool was a very wealthy seaport and had a large number of gentry involved in shooting. Information about the company is very sketchy but we do know the name was acquired by W. Richards of Birmingham. Unfortunately all records were destroyed in the Blitz so no use contacting Richards.
Here is a photo of Castle Street circa 1875. Their showroom was in the hazy distance about a block this side of the city hall, seen at the far end of the street. Pretty uptown digs for them.
Also, here is a link to a previous discussion of the company.
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=205973&page=all