High pressure building over northwest Europe is steering warm, humid air northward into southern England, supporting daytime maxima near or above 28°C in London on June 21 under mostly sunny conditions. Recent Met Office guidance and ensemble runs highlight a tight cluster around 28–29°C as the most likely peak, with modest model spread arising from uncertainties in cloud timing, boundary-layer mixing, and any weak frontal influences that could shave 1–2°C off the maximum or allow brief excursions toward 30–31°C. Official observations and climatological baselines place late-June London highs near 22°C, so the current setup represents a notable positive anomaly. Traders are weighting the latest high-resolution model updates and any revisions from the Met Office or ECMWF, which will narrow resolution criteria ahead of the 24-hour observation window.