Hunch. The monthly figures already put July and August so far above the spring months that September would have to fall by about 0.8% for the quarter to come in at 0.4% or less.
The 'beat April to June' market resolves on the same ONS bulletin: YES if the headline figure for July to September is 0.5% or more, which is above the 0.4% first estimate for April to June, and NO if it is 0.4% or less, whatever revised figure for April to June appears that day. The same rules for a moved release and for void apply, 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.