Over the past 18 months, the gap between UK and Japanese policy rates has narrowed sharply—by about 165 basis points since mid-2024—and is poised to compress further. Markets widely expect the Bank of England to trim rates by another quarter point to 3.75%, while the Bank of Japan is set to raise its policy rate to around 0.75%. On paper, a narrowing rate advantage should have weighed on sterling against the yen.
Instead, the opposite has happened. The pound has gained more than 1% against the yen over that period and has rallied roughly 5% since mid-year, reaching its strongest level versus Japan’s currency in 17 years. The message from markets is clear: interest rate arithmetic alone is no longer enough.
Part of the explanation lies in the yen’s prolonged weakness. Since the pandemic, Japan’s currency has fallen dramatically, with the Bank of Japan’s real effective exchange rate index down around 30% to its lowest level in more than half a century. The slide—politically sensitive in Tokyo—has often been portrayed as a byproduct of policies designed to re-ignite inflation. But this is not just a yen story. Sterling’s own real effective exchange rate has risen about 10% since 2020 and continues to edge higher this year, indicating genuine pound strength as well.
Bond markets reinforce the same puzzle. Short-dated yield spreads between UK gilts and Japanese government bonds have halved since mid-2023 and are now back near levels seen before Britain’s ill-fated 2022 budget. Yet over that same period, sterling has climbed roughly 14% against the yen. Inflation-adjusted yields tell a similar tale: real five-year spreads have narrowed by around 60 basis points, while the 30-year yield gap—where much of this year’s bond market volatility has played out—has shrunk by about 120 basis points since January. Still, the pound continues to advance.
With rates and growth offering few clear answers, fiscal policy has come into sharper focus. Despite political noise around last month’s UK budget, Britain is moving toward tighter fiscal settings. Japan, by contrast, under its new prime minister Sanae Takaichi, is embarking on yet another round of government spending. Those opposing fiscal paths help explain why the two central banks are diverging now—and why investors may be assigning a growing risk premium to Japan’s already vast debt burden.
Memories of Britain’s own 2022 crisis may be instructive. Then, markets quickly penalized unfunded stimulus plans by selling sterling and gilts. Today, investors appear more concerned that Japan’s renewed fiscal expansion could eventually constrain the Bank of Japan’s ability to tighten policy meaningfully.
The clearest evidence comes from capital flows. Foreign investors own nearly a third of the UK gilt market, and Japanese institutions account for close to a third of that foreign ownership. In October alone, Japanese funds bought the largest amount of UK government debt in more than four years. With nearly a two-percentage-point yield pickup on UK 30-year bonds and roughly 270 basis points on two-year paper, gilts remain attractive even as expectations for Bank of England rate cuts have grown.
At the same time, political turbulence at home has encouraged Japanese investors to step back from their domestic bond market, reinforcing the outward flow of capital. Those investment decisions—not the direction of policy rates in isolation—appear to be doing the heavy lifting in the pound–yen exchange rate.
In the end, the sterling–yen story is a reminder that currency markets evolve. Yield spreads still matter, but they now compete with assessments of fiscal discipline, debt sustainability, and the gravitational pull of global investment flows. For now, those forces are keeping the pound aloft, even as the traditional rate calculus points the other way.