The Earth's axis is tilted and this creates the different seasons. Each half of the Earth is called a hemisphere and Europe (where the UK belongs) is in the northern hemisphere. When this is tilted towards the Sun, it is summer in Europe. Have a look at the four seasons diagram below. The equator separates the Earth into the northern and southern hemispheres.
When the northern hemisphere is tilted away from the Sun, it is winter in Europe. The Sun is not very high in the sky at midday and days are much shorter than nights. The southern hemisphere is slightly closer to the Sun during its summer and that's why summers in the southern hemisphere are often hotter than ones in the northern hemisphere.
Contrary to what most people believe, the northern hemisphere is further from the Sun during its summer. The Sun still feels hotter, though, because its rays are much more concentrated than they are in the winter, as the Sun is higher in the sky. This is what makes the difference. Summer days are longer, so the Sun has more time to warm the Earth.