How to be a former prime minister

One of the world's most exclusive clubs - of people who used to run a country, but don't anymore - has received two new British members this year.

But what do former prime ministers and presidents do with themselves once they've left office? How do they continue to make a mark? And what might Boris Johnson and Liz Truss learn from their predecessors?

I've been examining this question for my new series on BBC World Service, How To Be A Former President.

What do you do first, though? Helen Clark, a former prime minister of New Zealand, has a simple answer to that one: "The first thing you do is take a long break," she says.

Almost everyone writes a book at some point, too - it's a way of filling the days, raising some money and, as another former prime minister told me, it is a way of ordering your thoughts - making sense of what you've been through.

And then there's giving paid speeches. It's often seen as a way to earn some easy money. But it can also be a way of wielding influence - as she travelled the world, Margaret Thatcher's speeches always pressed the case for her reforms, her view of the world.

On the Labour side, recent PMs have followed very different models: seeking to affect policy on the global stage.

But the focus of his post-premiership is on his role as UN special envoy for global education.

These roles can be a bit hard to pin down - being a special envoy can be a bit of a nothing job. So Mr Brown focused the question of funding education in low and middle income countries. He set up a commission to put some numbers on how much money is needed - and roped in friends from his time in office like former Australian Prime Minister Julia Gillard, CEOs like Jack Ma of Alibaba, education activists like Nobel prize winner Kailash Satyarthi, and even the popstar Shakira.

"There are global problems that require global solutions," he says.

"None of the major problems we have… can be delivered without people coming together to formulate responses and solutions that are global in nature. And education for me is the key that unlocks so many of the goals that we have set ourselves as an international community."

Erna Solberg, a former PM of Norway, who chose to fund much of the commission's early work, says working with Gordon Brown had several obvious upsides.

"I had the feeling that it would be easier for him to reach out to governmental officials, politicians in other countries on this agenda," she explains, "because he already was a very well-known person and a politician that had a lot of contacts."

The key, she says, is that as a former prime minister there is a big difference between Mr Brown and how other lobbyists or experts are received by people at the very top of government. "It's a sort of a courtesy that you also talk to former prime ministers if they try to reach out."

People in the sector acknowledge Gordon Brown's impact, and certainly welcome the visibility he's brought to the cause - but they don't always agree with his focus.

Tony Blair is another influential former PM. Mr Blair's early post-premiership was dogged by controversies over how much money he earned, and where he earned it - there's a whole book called Blair Inc about his business empire.

But in 2016 he closed it down, and his efforts are now focused on the Tony Blair Institute for Global Change.

Mr Blair's institute has a variety of programmes on British politics, and he regularly intervenes in political debates, but perhaps his greatest influence today is felt outside the UK. The Tony Blair Institute is currently advising 17 African governments on how to deliver on their priorities. Close observers may detect an echo of his own premiership, in which a focus on delivery and establishment of a Delivery Unit were hallmarks.

What is most remarkable about this work isn't so much that he's advising all those governments, though, but how. Because Mr Blair's advisers are inside them - sometimes inside presidential or prime ministerial offices. Lazarus Chakwera, Prime Minister of Malawi, spoke at once recent event hosted by Mr Blair's institute.

"In the last few months what we've been doing in pushing this, is to make sure that we institutionalise the delivery system," he told the guests. "The issue is delivery. It must happen, and it must happen on a daily basis." Mr Blair couldn't have put it better himself.

Boris Johnson and Liz Truss are both only months out of office, and still MPs, so perhaps it's no surprise they haven't yet set off on a shiny new career. But if they do, there are plenty of other models where they could seek inspiration.

Former Irish President Mary Robinson stood down early in order to take up a job as UN High Commissioner for Human Rights. Since then, she's been extremely active on questions of what she calls climate justice, and now chairs a group of former leaders called The Elders.

Both international organisations and members' clubs are popular ways for these former leaders to exert influence. The Elders had a big role in promoting debate about ending child marriage, for example.

And then there's Helen Clark. She also went on to run an international organisation - the United Nations Development Programme; and also now runs a members' club - the Global Leadership Foundation.

She is active on a huge range of issues, and I bumped into her recently at the Clinton Global Initiative, a giant shindig, founded by former US President Bill Clinton, for the great and good to come together and make promises.

She goes to a lot of events like that one, so I asked her what she aimed to achieve.

"People want to have hope," she replied. "Without hope the people perish, as the old saying goes. And the people are overwhelmed by the scale of the challenges.

"I mean, we live in a world that's terrifying in many ways. It's hard for people to take the view that actually out of crisis you could find some opportunity to break the mould and get things done."

For recent prime ministers like Boris Johnson and Liz Truss, looking for opportunity in the crises facing the world at the moment might not be a bad place to start life after office.