What makes Britain's weather so distinctly variable?

According to BBC News Magazine, "Britain is under an area where five main air masses meet. An air mass is a large body of air that has similar temperature and moisture properties throughout.".

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In the UK they are either polar or tropical, depending on where the air mass originated; they are also divided into maritime or continental, depending on whether the air has passed over land or sea. They come from all directions and can bring all types of weather.

When they meet it creates a weather front. The air masses fight it out and the one that wins dictates the weather. There is a sixth air mass, the returning Polar Maritime, which is a variation of the Polar Maritime.

Source: BBC News