Humidity is a measurement of water in the air. The UK is famously a very rainy country. This is what makes it humid over here. When you sweat, even in just the mid-high twenties, it just doesn't evaporate, it's got nowhere to go because they're already too much water in the air for it to be a quick process. And it's the evaporation of sweat that wicks away heat from us and cool us down, without that you just stay hot with liquid on your skin that's also getting hotter. This is what I'm describing and why a less humid country in the 40s has the possibility to feel more bearable than the UK in the 30s