Proof of legal stay requirement

I am a Dutch national who lives in England for the past 8 years. I have studied and worked for past 8 years. I am in the process of applying to visit China and Hongkong. One of the requirements is that i need to show proof of legal stay. I didn't apply for any residence card or my permanent residence, but my legal status is implied as long as i exercise my rights. How do i proof this when i apply for the visa?
Thank You
Regards