Near the beginning, Hengist and Horsa are watching a young cave couple dragging each other around. Hengist is pouring Horsa a drink and you can see him pour, raise the jug, then Horsa takes a sip from his cup. All this time it is patently obvious that no liquids are in the vessels.
If the props and sets on 'Carry On Cleo' look to be unusually lavish, this is no surprise. They were not made for the movie. Producer Peter Rogers just seized the opportunity of re-cycling the props and sets from the famous Richard Burton/Liz Taylor movie (the very same movie that they were sending up, incidentally). The props and sets had been left behind at Pinewood Studios when production of the Burton/Taylor "Cleopatra" moved to Rome.