My understanding with Uncle Sam is that it's like with Dracula. Dracula is in the public domain (in America, not sure about the UK), but each comic adaptation of Dracula is unique to itself so you can't do a version of Dracula that resembles those versions of the character, nor can you do anything too similar to the Universal Pictures Dracula. But you can do your own adaptation of Dracula--so long as you find a way to do the character that qualifies as unique and doesn't borrow too much from any of those others.
So with Uncle Sam there have been several and there were several versions of Uncle Sam during WW II. But the Quality character is their own trademarked version of the character. The copyright has lapsed, but DC still owns the trademark on the Quality Uncle Sam. Yet if you create your own Uncle Sam that's different enough from the Quality version, then that should be okay.