The temperature of the air arriving over Britain plays a large part in how hot it feels. Recently, air from the Mediterranian has been moving over Britain. It's already warm to start with, and though it cools as it moves north, the sun heats it further. If we had air moving down from the Arctic Circle, it could be just as sunny , but the days would feel distinctly cooler.
Humidity has a lot to do with it too. Moisture in the air can act as an insulator, keeping cool air cool despite full sunshine, and warm air warm, giving us those hot muggy nights. Air moving up from the Azores tends to be moist, while air coming in from the Continent tends to be drier.
If the air is very dry, the sun will feel much more intense during the day, but at night the heat escapes into space. This is how some deserts can experience incredibly high daytime temperatures, yet be frosty at night.
The heat in the tropics is regulated by air movement. The hot moist air rises, and moves away toward the poles - the heat is taken with it.
So the day's temperature is determined by 1) the sun's position, (High in summer = more direct and hot, low in winter = less direct and cooler). 2) Where the air over Britain is coming from, (warm or cool, and moist or dry)