I am one of the people in this situation. Edexcel screwed up my History grade, and Cambridge weren't keen on the AAB I got on results day. Once I'd explained to the school there is a priority remark service (they didn't know!) I managed to get it remarked and my grade went up. This was past the 31st August deadline, so they asked me to wait a year. Hence the gap year.
It is my understanding that they are not legally obliged to take you after 31st Aug, but there is an obvious moral obligation!