The Profile Morskie devoted to HMS Cornwall has a nice picture of her from the stem looking aft, in which the patterned non-slip steel deck can be clearly seen. You can see the picture here at the bottom of the page:
The non-slip pattern on the steel deck can be more clearly seen in the actual picture.
Also, circumstantially, other units in the class such as Berwick, Kent, and Suffolk had steel foc's'l decks. I believe, however, that Cumberland's foc's'l deck was planked.