Well - there is one consideration here - BA might be selling these seats to people starting their trip in Europe - not all BA's customers are UK based. With the exEU deals you still fly on the same aircraft as the UK-based passengers, but paying (sometimes) substantially less. And the difference is bigger than just the APD.
My suggestion to oil-man would be to look at BA fares out of MAD or CDG. We did Italy last year, and it worked fine, plus saved us more than £1k.