UCAS parental educationWatch
You can select this on the UCAS Apply form.
But how do UCAS or the universities verify what you put is correct? Is it based on trust? As how else could you possibly prove they haven't been to university.
Generally- if it’s important then it will be checked. The universities I know of with that as a requirement for medicine foundation will require a letter from a referee confirming parental education.
While it’s possible to lie the consequences of getting caught are severe. And the chances of getting caught are pretty high.