Quite a few places actually
all you had to do was search...
https://www.google.co.uk/search?q=london+shotgun+range&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&ei=3of3WPnLDsXW8ge54K2gAg#q=london+shotgun+range&rflfq=1&rlha=0&rllag=51561451,-158061,12890&tbm=lcl&tbs=lrf:!2m1!1e2!2m1!1e3!3sEAE,lf:1,lf_ui:1