Hunch. When ONS last revised its figures in May 2026, one year went down by 14,000 and another went up by 15,000, so the change to 171,000 may be small and could go either way.
The revision market resolves on the same release: YES if the ONS now puts net migration for 2025 above 171,000, in the bulletin or its main table, and NO if it stays at 171,000 or goes lower. Backups and a moved date work as for the below 171,000 market.
We looked for cases where the ONS gave its regular estimate of UK net migration for the latest year, and compared it with its first figure for the period before. The ONS changed its method in 2021 and says older and newer figures should be compared with caution. We found eight such releases from November 2022 to May 2026. The new figure was lower three times, and those were the last three.