The 30-year never came near 5.40 in the window: the lowest reading on the Bank of England's daily nominal spot curve between 31 July and 7 August 2026 was 5.78 per cent on 5 August, 38 basis points above the line.
The 'close below 5.40%' market resolves YES if any daily reading in the same series before 7 August 2026 is below 5.40%; otherwise NO at expiry.
This is the fourth spike in four years. Each time, UK long-term borrowing costs jumped to their highest in decades after a scare about government finances or inflation. The three spikes before this one all eased within about two months. But each peak was higher than the one before, and the level they eased back to keeps rising.