The Bond of Power and the Power of Bonds
Yield Curve Politics: The (not so) silent kingmaker of 10 Downing Street
Here’s something they don’t teach at the Oxbridge PPE programmes that produce most British Prime Ministers: you don’t actually get to govern unless the bond market lets you.
Not the House of Commons. Not your cabinet. Not the press lobby, your internal factions, or the increasingly creative polling methodologies that tell you whether the public prefers your energy to your opponent’s vibe. The bond market. The £2.7 trillion market in UK government gilts - which is, for all intents and purposes, the financial world’s running commentary on whether it believes a word you’re saying as a PM (or PM challenger).
As Keir Starmer’s premiership enters what the newspapers are diplomatically calling “a difficult period” (read: his poll numbers have fallen off a cliff and the party is doing that restless energy thing where everyone’s suddenly very interested in the rules for leadership challenges following the local elections recently), the usual Westminster machinery has kicked into gear. Potential successors are gearing up with more conviction than usual. Allies are briefing journalists. Everyone is very busy not-quite-running for a job that isn’t quite vacant yet.
Fine. Normal politics. But here’s what the challengers need to understand before they start printing business cards with “10 Downing Street” on them: the gilt market will have opinions about you. Loud ones. And unlike a parliamentary select committee, it won’t give you three weeks to prepare your answers.
Why gilts, and not, say, the FTSE?
This is actually an important question and not enough people ask it. The instinct, especially for politicians whose economic literacy was formed in an era of cheap money and benign markets, is to watch the FTSE 100 as the national economic mood ring. It’s on the news when the markets are at an all time high (or low). It has a number. It goes up and down in a satisfying, legible way.
The problem is that the FTSE 100 is basically a collection of multinational companies that happen to be listed in London. When sterling falls, FTSE earnings (many of which are denominated in dollars) mechanically go up. So you can have a genuine UK sovereign credibility crisis - the kind where serious people in serious buildings are having very unserious conversations about emergency liquidity, and the FTSE might be fine. Slightly up, even. This is not a useful signal.
Gilts are different. When you buy a 10-year GILT, you are making a very specific bet: that the British government will be solvent, competent, and broadly not chaotic for the next decade. The yield on that gilt is the market’s real-time price for that bet.
Low yield = we believe you, carry on.
High yield = we have concerns, please explain yourself.
And unlike a parliamentary vote of no confidence, the gilt market runs every single business day. There is no recess. There is no prorogation. It just... prices things. Continuously. In public.
The 45 day internship of Liz Truss
You know the story. But it’s worth telling it again in a specific way, because the lesson most people took from it (”don’t do unfunded tax cuts”) is the right lesson but slightly incomplete.
In September 2022, Truss and Kwasi Kwarteng stood up and announced £45 billion of tax cuts in their mini budget. Fine, tax cuts are a legitimate policy choice. What they also did, with what can only be described as remarkable confidence, was skip the Office for Budget Responsibility (OBR) forecast that would have independently stress-tested the numbers. This was not a small thing. The OBR exists precisely to give gilt investors an independent anchor - a document they can point to and say “okay, here is what independent experts think happens to the deficit.” Without it, investors had nothing to price off except vibes and political conviction, which are not, historically, great collateral.
The gilt market’s response was immediate and, by the standards of sovereign debt markets, genuinely dramatic. Thirty-year gilt yields moved by amounts that traders described, with some understatement, as “unusual.” Sterling fell toward parity with the dollar. Pension funds, which had been running liability-driven investment strategies using gilts as collateral, started getting margin calls that threatened to turn into a full-scale liquidity spiral. The Bank of England (BoE) had to step in and buy gilts. Not as monetary policy. As financial stability emergency response. To fix a problem the government had created. Twelve days later, Kwarteng was fired. A month after that, Truss resigned.
The gilt market had, in no gentler terms, sacked a British Prime Minister.
And here’s the thing: it didn’t do it because it had ideological objections to tax cuts. Bond investors are not, as a group, closet socialists. They did it because the numbers didn’t add up and the government had made it very clear it didn’t particularly care whether they added up.
So what does this mean for whoever comes after Starmer, if at all?
The UK’s fiscal position right now is not comfortable. Debt-to-GDP is north of 95%. The OBR’s famous “fiscal headroom” has a habit of disappearing the moment anyone looks at it carefully. Growth is sluggish. The gilt market is not in crisis, but it is paying attention.
Any successor who comes in swinging with big spending commitments and a casual attitude toward how they’re financed will find the gilt market waking up very quickly. Any successor who overcorrects into aggressive austerity will likely strangle the growth that makes the debt trajectory sustainable, and gilt investors, who are quite good at modelling debt dynamics, will notice that too.
The viable path (which is genuinely narrow) is what you might call gilt-compatible ambition. Growth-oriented but credibly costed. Deferential to the OBR (not because it’s always right, but because it’s the institutional anchor the market trusts). Clear in communication to debt markets about the rules of the game. Rachel Reeves, for all the political noise around her, has understood this constraint more clearly than most. Any successor who is smart will either keep her or find someone who operates with equivalence in this regard.
The yield curve is the biggest a learning curve. It just charges very high tuition.
A brief note on Donald Trump, who also figured this out
The POTUS’s second term has featured an ongoing, unacknowledged, and quietly fascinating accommodation of Treasury market signals. The sweeping reciprocal tariff announcement in April 2025? Paused for ninety days, almost precisely as ten-year Treasury yields pushed toward levels that would have made US corporate debt refinancing genuinely painful. The Iran escalation rhetoric? Modulated when dollar funding markets twitched. The threats to Federal Reserve independence? Walked back when bond markets started pricing in the kind of institutional chaos that makes foreign holders of US Treasuries nervous.
Trump would never say “I listened to the bond market.” But he did.
Even the proclaimed ‘most-powerful person in the world’ operates within a constraint set, and at the top of that constraint set, above Congress, above the courts, above polling, is the cost at which your government can borrow. Push that too high and nothing else works. You can’t fund your agenda. You can’t cut rates to juice growth. You can’t run the deficits that pay for the things your voters elected you to do.
Trump, whatever else you think of him, is a transactional pragmatist. He found the constraint and he worked around it. That’s actually the correct approach.
The bottom line
Power, in an open capital market democracy, is always partially borrowed. And borrowed from people who will re-price it if you do something they don’t like.
The constitutional theory says that a Prime Minister serves at the confidence of the House. The practical reality is that they also serve at the confidence of whoever is buying UK sovereign debt.
For anyone in Westminster currently doing the quiet maths on a leadership challenge, the question isn’t just: can I get the votes? It’s: what happens to ten-year gilt yields when I announce the challenge or the morning after I win?
If you don’t know the answer to that, if you haven’t thought about it, if your team hasn’t modelled it, if you’re assuming it’s a detail to sort out later, then the bond market will sort it out for you.
It’s quite good at that.