As Britain prepares for its most unpredictable election in decades, the country’s method of selecting a government is coming under nearly as much scrutiny as the candidates or their political positions.

British elections generally produce clear winners in Parliament. But this time, polls suggest that the voting Thursday is unlikely to result in a legislative majority for any of the three main parties — the Conservatives, Labour or the Liberal Democrats.

That would be a ho-hum outcome in many other parliamentary democracies, where the parties would sit down to hash out a governing coalition. But in Britain, where there is limited precedent for so-called hung Parliaments, the prospect has prompted a frenzy of tactical feints and parries this week as the three parties position themselves for the maneuvering that could follow....