It took until the last possible day, but overnight the British government finally got approval for its legislation for a referendum on preferential voting. The UK will vote on 5 May, in conjunction with Scottish, Welsh, Northern Irish and local elections, in only its second-ever nationwide referendum.

It’s a lot of effort for not a huge reform, but in a country so attached to its traditions the prospective change evidently seems larger than it is. If approved, the referendum will replace the first-past-the-post or simple plurality voting system with an Australian-style preferential system (referred to in Britain as the alternative vote, or AV) — specifically, optional preferential, as used in state elections in New South Wales and Queensland.

The unfairness of the current system is manifest. In 2005, Labour won a majority of more than sixty seats with just 35.2% of the vote. Last year, Labour and the Liberal Democrats won 52% of the vote between them, but were well short of a majority of seats. The Lib Dems had 23% of the vote but won less than 9% of the seats; back in 1983 (as the alliance of Liberals and Social Democrats) they managed just 3.5% in seats with more than 25% of the vote.

Moving to AV would improve things, but not by very much. The Lib Dems would gain, but not to the extent that their share of the vote (which is dropping precipitously in the polls in any case) would suggest. So would whoever ends up getting the greater share of Lib Dem preferences; historically that would usually have been Labour, but with a coalition government that is no longer at all certain.

A change to a genuinely fair system — such as German or New Zealand style proportional representation — is not on offer; AV was the best that the Lib Dems could get in their negotiations with the Conservatives. And even passage of AV is by no means assured, although opinion polls have generally put the “yes” vote in the lead.

The Lib Dems, of course, are the strongest supporters of change. The Labour leadership is also backing the “yes” case, but its voters and MPs are divided. The Greens and other minor parties also support change (except the neo-fascist BNP), although most, like the Lib Dems, would have preferred PR. The main opponents of AV are the Tories, and the result will largely depend on just how hard they are willing to push the “no” case – given that doing so will mean attacking their own coalition partner.

There is an obvious comparison here with the 1999 republic referendum in Australia; a proposal for change being offered by a government whose leaders are mostly opposed to it. In each case, it is easy — and tempting — to exploit public ignorance with a scare campaign.

One might think that AV, a limited reform and one that has run successfully in Australia for more than 90 years, hardly compares with revising the whole constitution to abolish the monarchy. But that would be to underestimate the degree to which those accustomed to first-past-the-post can simply fail to see any unfairness in it and fail to understand the merits of any alternative.

Because very few people take an interest in the details of electoral systems, it’s easy to mislead voters with such red herrings as compulsory voting (not being discussed), the cost of electronic counting (also not being proposed – if we can count by hand, there’s no reason why the British can’t) and six-week delays for results (not an issue in Britain, since postal votes have to be received by polling day). As Antony Green (put it earlier this week, “Many Australians would be amazed to read the UK press and discover the terrible iniquities heaped upon them by the Alternative Vote.”

The proponents of change have just eleven weeks to repel such attacks and convince the British electorate that this is a change worth making.