To start with, neither FPTP nor AV have the first bloody thing to do with ‘democracy’. We don’t have ‘rule by the people’. We are ruled by a largely self-selecting political class, and are allowed to shake the kaleidoscope every 4 years or so, before watching things continue more or less as before: gaudy and childish. The referendum will be between two different methods of shaking.
One of them, FPTP, disproportionately rewards parties with a record of previously winning, makes only a small percentage of seats competitive and amplifies swings in voting patterns to such an extent that a small cabal of party apparatchiks usually get to dictate national politics for the next four years. The other reduces entry costs for new political parties, does not reward established parties, and makes it much harder for a single political party to win an absolute majority, though it isn’t proportional.
As far as I can see, the only argument for FPTP is: ‘I want my chosen political cabal to wield absolute power in 5 years stretches and not have to bargain with other political interests’. Good to see an issue separate the dyed-in-the-wool atavists from the lukewarm reformers.