Indicate the disability on the UCAS form.
What others have suggested re the notification coming from the School is really really the best (only) way. The sentence suggested upthread is ideal and would be very useful to me as an academic trying to make an evaluation of an applicant's ability to do well in my course:
Due to very late diagnosis of xx, for which 25% extra time will now be given in exams, I feel Tarquin's GCSE and AS results do not reflect his true ability, and that had this support been in place when he took his ASs I would be predicting A2 results of A A A, instead of AAB
As an academic within a university department, if I received a letter from a parent, but nothing from the school in the school reference, or as additional documentation, I'm afraid my first response would be to discount it. Because, well ... it's coming from a very non-objective and non-expert source, I'm afraid!
However, fair admissions codes would mean that I'd probably refer it back to our University admissions office, for their clarification. They'd contact the school, I imagine, not the parent -- or they might contact the educational psychologist who has diagnosed the disability. .
I hope you understand that within Departments, we really can't enter into dialogues with parents on these matters.
But because of its late diagnosis, if the school didn't include something in the reference, I think you could ask tat they send an extra letter -- I've seen this done a lot. It goes NOT to the individual Departmental Admissions Tutor, please, please not! But to the University's central Admissions Office. As another posters says upthread, individual academics in Departments don't generally see the details of any declared disability. That's all dealt with centrally across the whole university, so there's parity. And central Admissions people have the expertise to make these judgements, which I don't have. What I do I have the expertise to evaluate whether a student is equipped academically to thrive in my course.