UCAS statements: Do you discuss what you want to do in gap year?
Why you want to do course, interesting work undertaken at school, extra curricular/responsibilities
Clueless really - what makes a personal statement stand out /be accepted/ particularly if you want to defer?
However, to try to give you a general answer to these specific questions: the MAIN thing in a PS I look for is an understanding of the subject to be studied, in a scholarly context, so I know that the applicant has some realistic idea of what s/he is applying to read at university.
So if, for example, it's Hamster Studies, where there are different approaches and levels of study, from the zoo keeper/demonstration/hamster training level, to the arcane theory of hamsters through the ages, then I'd not want someone to write: "All I have ever wanted to do is clean out hamster cages." They'd need to show that they understand what it is to study the subject at a high level, including the theory and history of hamster studies.
And IF extra-curricular studies are relevant to this, then mention them. But otherwise, I'm really not that interested, apart from seeing that this is an applicant who can manage to do the DoE Gold Award and get A grades. But if it's a university that's used to dealing with high-achieving young people, then we see that kind of achievement fairly regularly. So it's not going to sway us too much to be honest (well, not me anyway).
Ditto what is to be done in the gap year: we're increasingly aware of ways the socio-economic advantage filters through into gap year opportunities, so there may be issues there. However, if gap year experience is very specific and pertinent to the degree area I teach, I'd ask the applicant about that at interview -- we still interview at my institution ...
But all of this is only a consideration if the grades or predictions, plus GCSE results, are in order. As creamteas says, it may be better to apply post-qualification if you think retakes are going to be necessary. And I'd expect that the school's reference would explain the retakes, as well as the under-performing original results.