Hunch. The monthly figures already put July and August so far above the spring months that September would have to fall by more than 1% for the quarter to come in under 0.4%.
The 0.4% market resolves on the ONS bulletin 'GDP first quarterly estimate, UK: July to September 2026', due at 7am on 12 November 2026: YES if the headline growth for July to September against April to June, to one decimal place, is 0.4% or more, and NO if it is 0.3% or less. If the release is moved it resolves on the actual first estimate, it is void if none is published by 31 December 2026, and BBC News and The Guardian settle it if the ONS page is down.
We looked for cases where the Office for National Statistics published its first estimate of quarterly growth after the monthly figures for the first two months of the quarter had both shown growth. We found ten such quarters since monthly GDP figures began in 2018, from April to June 2018 to January to March 2026. In eight of them the first estimate was 0.4% or more, and in two it was lower.