In Australia we recently had a postal ballot to determine if the definition should be restored to "two people" rather than "a man and a woman".
Sample polls already knew what the result would be (~60/40 in favour) but our government spent $122 million on the non binding ballot because a few conservative politicians wouldn't allow parliament to simply vote.
Britain has a population 2.5 times that of Australia with an exchange rate about 0.6, so our ballot was equivalent to Britain's government spending about £230 million. A whole lot more was spent on pointless advertising by the opposing sides including millions by some churches that could have otherwise gone to aiding the poor.
At least you got something to show for it.