St. Paul's Cathedral is the place where Prince Charles and Diana were married.  It is beautiful, with wonderful byzantine style mosaic ceilings.  It is not steeped in the overwhelming amount of history and artifacts and memorials that Westminster has.  It's days are relatively recent. . .It took a direct hit to the dome during WWII.    

Also, Sr. Admiral Horatio Nelson, a WWII British war hero has an amazing memorial tomb down in the lower level tomb area.