Hi
I am not an admission tutor but I am a mother who has been through this once and I am part of a RG/1994 group uni and get called upon from time to time for advice by our admissions tutor. I am not a Scientist but DD is.
What really turns you off a personal statements is if they are obviously not written by the candidate and you really can tell (all sorts of clues, archaic or overly sophisticated language, business / marketing speak, self help psychobabble, inconsistencies, clear ignorance of the academic discipline as it is taught now, formulaic layouts etc ). I know plenty of mothers and fathers that have "redrafted" and "tweeked" DCs personal statements, only to be offended when the school changed it back to the pupils own attempt, and they were quite right to. My biggest advice would be to keep your nose out, if your school is hopeless then there are plenty of websites etc with common sense advice for your DC. Obviously badly written personal statements are also a turn off, by this age even if they have an SLD they should know how and be motivated to get that checked.
What really impresses is if the candidate has really done their homework about the course and then provides lots of evidence of their particular passion for the subject in the context of it's relevence to the course, and the qualities they will need to do well. This will be demonstrated if the applicant focuses on their own responses to what they have seen, read etc. So it is not impressive in itself that your parents have taken you around the planet and bought you every book ever written on the subject, and paid for you to join an expedition to the Galapagos. It will be just as impressive if you have persued your passion for your subject in far less glamourous ways, reading, lectures etc (even if only on itunes U) and say what you have learned, how you have developed your interests and your own opinions. If your son has been working with his Dad repairing electrical appliances and he has learned something that is relevent to the subject that he wishes to persue, both in terms of academic knowledge and skills, and personal qualities (work ethic, practical skills?) then he should put that in. Frankly that the activity might betray he is working class is no more relevent than if all the trips across the planet betray a pupil is wealthy, likewise the family connection. I think a university admissions tutor might be pleased to see that networks of family influence work for working class pupils too, since they are not naive about all these amazing internships in law, medicine, banking etc. some lawyer, doctor and banker offspring seem to manage!! Most of your personal statement should be given over to academic matters, extra curricular and activities out of school only impress in so much as they demonstrate that they have learned relevent skills such as teamwork, presentation skills etc. Also do not waste words on vague expressions of passion, anyone can say it has been their dream since they were 6 to read Physics but it is what they have done about it in the last two years of A level study that will make the difference. Most of all it has to be an expression of your child's particular passion for their subject and their particular qualities and as such there is no formula.
The place for any detail on UMS marks is the school's reference. I would expect them to mention the 100 UMS on one module as evidence that the A level course has not tested him to his full potential but to focus on the positive of his predicted grade. The same goes on contextual information on a pupils school, background etc. Personal statements are defintely not the place to waste words on excuses.