You don't have to mention your circumstances in your personal statement if you don't want to – even if you flag your circumstances in the application question.
However, some people like to use this space to talk about how their experiences have influenced their subject choices or chosen career. In some cases, students will find they have developed strengths and skills they can apply to their studies. With the help of students who are estranged from their parents, UCAS has created a personal statement guide for estranged students to help you identify these skills and strengths and explain how they're applicable to your studies.
If you do choose to mention your circumstances in your personal statement, please also use the application question – student support staff won't see your personal statement, so you may not get the right support if you don't answer this question too.