How would you change the voting system in case of a referendum? It was the easiest possible case, for or against. There’s no advantage to voting “for” when you’re against or vice-versa. You could require a supermajority, but on which basis? Because you don’t like the outcome? Setting arbitrary thresholds would just be indirect democracy with extra steps.

Now of course the UK should get rid of FPTP in favor of another voting system, but that doesn’t have anything to do with the referendum.