Complex sentence: The British Football League is a "proposed" association football league which would involve the merger of the Scottish, Welsh and possibly the Northern Irish football league systems into the larger English football league system.
Split into simpler sentences: The British Football League is a "proposed" association football league in the United Kingdom that would incorporate clubs from across the Home Nations. Its creation would involve the merger of the Scottish, the Welsh, and possibly the Northern Irish football league systems into the larger English football league system.