According to the admissions code, only using an interview would be illegal, which is very different from the situation in an independent school.
I think if you read the whole Admissions Code, boring, I know, I think you will find the answers to your questions.
This is parallel to the situation in university admissions where sometimes a student gets onto a course with lower grades than predicted. The apparent unfairness of this is what has stimulated discussions about only allowing uni applications to be made after A level grades are known, as in countries abroad.
But if this were to happen it would mean an almighty scramble to apply and be given a place after GCSE results are known which would be about 2 weeks before term starts, which unfortunately is not really seen as possible now. Or GCSEs would have to be taken in the spring term, which a lot of people would complain about and then what would the students do during the summer term?
I get the feeling that you are angry about this and kicking against the pricks rather. Do you honestly want DC to go to this school? If so, you know now what he/she has to do.
I think you need to make up your mind(s) about what you want and/or try looking at other schools in the area if you honestly think he/she may not get an offer.
Looking up judgments won't change anything and, as my research has shown, it looks as if they actually are not doing anything illegal. If you read to the end of the Code, you'll see that if they were, you and probably lots of others in your position would probably have appealed to the adjudicator about it. If it is that good a school it is more or less certain that they know the law and are complying with it, especially if they are such a high profile school. Sorry and all that, I can tell you are upset.