ONS marks non-geographic postcodes (no grid reference) with a placeholder latitude of 99.999999. The "Latest Centroids" export shipped ~12,789 such rows, which were imported as real postcodes pointing at lat 99.99 and would poison nearest-station distance maths. Drop them at ingest, with a test. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>