This early piece of artwork was preserved by the series' composer, Paul Lewis. It appears to have been produced before filming began, though clearly the main characters had been cast. Also, it had already been decided that Arthur should ride a white horse, and the font used in the series had been designed.
Arthur is seen carrying a shield of a type he only uses in the first episode - a round shield with a boss, which he later identifies as being of Saxon design. Also, we never see a Saxon longboat as shown in the poster - in full sail - in the series.
The picture of Kai - with a stubbly beard, which he doesn't wear in "Arthur of the Britons" - must have been drawn from stills of Michael Gothard playing Hansen in "The Last Valley", with which the producers were familiar.
Michael Gothard as Hansen