What actually causes blockages? Could I have prevented it?

Inside the property, the usual causes are:

Fat, oil and grease poured down kitchen sinks. These cool and harden inside pipes, narrowing them over time.
Hair and soap binding together in bathroom waste pipes.
Wipes, cotton products and sanitary items flushed down toilets. These do not break down like toilet paper and are a leading cause of blockages.
Food waste, including coffee grounds and rice, which swell or compact.

Outside, drains face a wider range of problems:

Leaves, silt, twigs and debris washing into gullies
Tree roots growing into pipe joints in search of water
Scale and sediment building up inside older pipes
Collapsed or displaced pipes from ground movement or age

In hard water areas, including much of London, limescale builds up faster. Calcium and magnesium deposits gradually narrow the pipe, slow the flow, and make blockages more likely.