Complex sentence: A short-haul domestic flight is commonly categorized as being no longer than or taking under one and a half hours to complete, meaning that all domestic flights within relatively small countries like the United Kingdom are short-haul.
Split into simpler sentences: A short-haul domestic flight (where the destination airport is in the same country as the departure airport) is commonly categorized as having a flight length taking under one and a half hours to complete. By this definition, all domestic flights within relatively small countries like the United Kingdom are short-haul.