The humidity kills you in the UK. I'm a fairly well-travelled person from a very young age, having travelled extensively throughout Europe and North Africa. A 35 °C in California is completely different to a 35 °C in the UK. Watch what an American says about how much hotter than anything he's ever experienced in the USA.
Higher Humidity (The "Feels Like" Effect) Because the UK is an island, the air brings in more moisture from the Atlantic. High humidity prevents your sweat from evaporating, leaving you feeling sticky and preventing your body from cooling itself. When it is 35°C in the UK, the "feels like" temperature can soar well over 40°C (104°F). In contrast, a 35°C day in California or Texas is often a "dry heat," allowing sweat to evaporate instantly.