Devil's Advocate here! If one was a 25 year old applying for licences and had been orphaned at age 6 months, would one be obliged to have knowledge of, or to search for medical details, of one's deceased parents? Would their GP be entitled to give this information to you if you asked him and he still held it or had access to it? Just a legal quibble really!
This is my situation exactly. I was adopted as an infant. I don't know what the situation is in the UK, but here in Canada, when I am asked about family history, I simply tell the examiner that I was adopted as an infant. He puts that down on the form, and that's the end of it.
It really is only so that the examiner can watch for signs of similar conditions to what your parents had, for example heart trouble if your father had a history of same, diabetes, etc. It won't "fail" you unless you happen to show signs of the same condition, but will enhance vigilance for the same conditions.
Beech.