Very big because at the end of y12 you sit UCAS exams which give you predicted grades for uni so if you slacked in Y12 you'd be worried about picking up all the old content and get a reality check by y13 and have to juggle between Y12 and Y13 content which is very in depth
yh thats why you should finish all the y12 and y13 content by the end of y12 or before that preferably