The duty isn't that much. About 50p on a pint maybe. That's probably wiped out by the economies of scale on alcoholic vs non-alocholic beer at a similar price point.
The joke priced beverages in pubs, based on cost of supply, are post mix fizzy drinks. It's almost like they charge what the market will bear and has little to do with cost.
How come non alcoholic beer isn't a lot cheaper? There's less tax/duty surely?