The fat and grease make a real mess that is difficult to clear.
From the TW site:
In total, we clear around 55,000 blockages from our sewers each year. The majority of these are caused by cooking fats and oils, which congeal in the sewers forming a thick layer around the pipe. This prevents sewage from flowing and can cause it to back up, especially when products such as wet wipes and sanitary item – which were not designed to be flushed - mix with the fat and set hard forming a solid obstruction.
Hey, sewer abusers....whoop....whoop....whoop.
"Working in very small sewers clearing fat and blockages. This makes the job very difficult for us, and it also seems so unnecessary as these problems could be avoided if people could just avoid pouring their used cooking fat down the sink and put it in the bin instead." - Chief flusher Rob Smith, on the worst part of working in the sewers