Explainer: Why has Britain gone through seven prime ministers in 10 years?

Explainer: The joke goes, how do you know when it's time to check your smoke alarm batteries?

Answer: When the United Kingdom names its next prime minister.

British PMs have had a shockingly short shelf life for the last decade, and Britain has gotten very familiar with the ritual of a dejected leader standing outside 10 Downing Street's melancholy brick walls as Sir Keir Starmer did on Monday, announcing he would step down as Labour Party leader and PM after barely two years.

Starmer's resignation sets up the UK for its seventh prime minister since July 2016, when David Cameron's six years in office - which now seems a positively leisurely term - came to an end. Not a single prime minister has served a full term in office since.

Former Greater Manchester Mayor Andy Burnham appears poised to take up the somewhat poisoned chalice, but will he succeed where a crowd of leaders haven't - and what's the reason for the revolving door of leadership, anyway?

Why can't the UK keep a PM?

Like New Zealand, Britain's parliamentary system doesn't require a general election to change prime ministers. If a party loses confidence in its leader, a PM may resign before a humiliating leadership vote forces them out. And in the last 10 years, this has happened five times.

"People on the streets I think are rather sceptical about British politics at the moment," the BBC's correspondent Rob Watson told RNZ's Morning Report.

"I mean, that's why we are where we are because the British voters seem to get very cross and very impatient with prime ministers and politicians very quickly. That, then, leads their parties to get rid of them."

There are many factors driving the turnover - lingering fallout from the Brexit vote and 2008 economic crisis, as well as rising calls for electoral reform of the UK's first-past-the-post system. That system has seen results such as Labour receiving 33.7 percent of the vote in 2024 yet getting a massive 174-seat majority in the House of Commons.

"Most British people want electoral reform and have wanted that for some time," said Robert Patman, international relations professor at the University of Otago.

"And the two-party system, the Conservatives and Labour, have largely resisted that trend. So there's tremendous disenchantment with the major parties."

University of Auckland global studies professor Chris Ogden said that Starmer faced heavy headwinds.

"Despite a large majority, Starmer inherited a country beset by deep-seated economic and social issues, and an impatient social media-fuelled public who expected rapid and successful change.

"The growing popularity of the far-right Reform party exacerbated these pressures, making his position appear existential for the country, his party and traditional politics".

Nigel Farage's right-wing populist party Reform UK has consistently been leading polls for more than a year.

British writer and academic André Spicer recently told RNZ expectations from leaders have created a cycle of overpromising and underdelivering that's causing shorter and shorter political careers.

"They realise very quickly that number one, you can't deliver on many of the promises which you've made. You're often told by a senior civil servant, you can achieve one, maybe two things as prime minister. And you've promised hundreds, if not thousands."

Paradoxically, Watson said while voter polls indicate people are fed up with the status quo in Britain, they're also sick of leadership changes.

"The voters still say they hate it when parties engage in internal squabbling and change."

So just how short have the UK's leader tenures been?

Since David Cameron, not a single prime minister has made it far past the three-year mark in office, led by Liz Truss's dubious honour of becoming the shortest-serving leader in UK history with just 49 days in total in 2022.

Theresa May: 3 years, 12 days

Boris Johnson: 3 years, 45 days

Liz Truss: 49 days

Rishi Sunak: 1 year, 255 days

Keir Starmer: 2 years, about 13 days so far

Is this all because of Brexit?

Well, kind of. It certainly seems to have jumpstarted the turmoil.

Cameron campaigned against the Brexit vote for the UK to leave the European Union and when that passed by a narrow margin, he announced his resignation.

Ten years on, Brexit's impact on the UK is being reconsidered and many economists say it's had a negative impact on Britain's economy.

"It's been a financial disaster, as many people predicted," Patman said, calling it "probably the worst policy decision since appeasement in the 1930s."

Cameron's successor, Theresa May, was also brought down by Brexit when she failed to secure parliamentary support for her plans for withdrawal from the EU.

"There's a clear trend to rejoin the EU," Patman said. "And that will happen, I think."

Boris Johnson, on the other hand, was humbled by a series of escalating scandals including parties at 10 Downing Street that violated Covid-19 lockdown rules.

His successor, Liz Truss, was quickly pushed out over controversial tax plans, and her lasting moment of viral fame was having her term in office compared to the longevity of a head of lettuce.

Rishi Sunak, the first prime minister of Indian heritage, took over for Truss but also shouldered the burden of Conservative Party turmoil. Instead of being rejected by the party, he lost his office when Starmer's Labour Party won a landslide victory in 2024's elections, returning Labour to Downing Street for the first time in 14 years.

So why did Keir Starmer fail when he had such a huge majority only two years ago?

The consensus is the 2024 election was more of a referendum on the Conservative Party's long run at the top than a whole-hearted endorsement of Starmer, and the first-past-the-post system gave Labour a big majority.

"Change begins now, and it feels good!" Starmer declared at the time.

But Starmer became increasingly painted with the image that he was ineffective and indecisive by the British press. His former adviser Peter Mandelson was revealed to be close to Jeffrey Epstein, and Labour's sweeping loss in local party elections in May sealed his fate.

"I think that Starmer, one of the reasons he fell, he's got a good intellect, but I don't think he's got good political instincts," Patman said.

Ogden said Starmer's charisma was also an issue.

"Without an easily digestible vision, combined with a lack of personal charisma, a series of policy u-turns reduced the Prime Minister's authority and control."

The lingering impact of Brexit also has played a role and debates over whether it should be abandoned.

"One of the reasons Starmer fell is Starmer just couldn't address this issue," Patman said, while noting that his likely successor Burnham has come out for electoral reform and is pro-EU.

Does Andy Burnham have any chance at success?

Watson said that while Burnham is regarded as a "sunnier, more relatable, obviously likeable" person than Starmer, he still faces serious obstacles, including the popularity of Farage's Reform UK.

"Based on what's happened to Keir Starmer and his five predecessors who all ended up being fantastically unpopular, logic would suggest that maybe Andy Burnham is going to suffer the same fate as the rest of them.

"I think that's because if you step back from it all, the challenges that Britain faces ... economic challenges, social challenges, public service challenges, I mean they're just not going to go away, Andy Burnham will be facing them."

Political parties like the Conservatives and Labour are "in deep trouble," Patman said, but he felt more optimistic about Burnham.

"I think (Burnham's) got a chance, but I think he's got to show some real courage," Patman said. "People like Farage are very powerful and very well funded."

"My hunch is that Burnham may well do well," he said, calling him a "street fighter."

"He's combative, and I think he's grasped the two issues that need to be addressed quickly, which is electoral reform, and actually being much more proactive with building relations with the EU."

Watson said that the problems facing the country run deep.

"Part of Britain's problem is a sort of cloud of gloom and despair and pessimism that's sort of descended on this place really since the financial crisis in 2008 and then exacerbated by 2016.

"Perhaps if you had a leader who was a bit sunnier maybe that might improve the weather more generally in a political sense. I mean, that's what we're about to find out."