"How would it have protected them?"
Like in any other modern country, they would have been registered as citizens on arrival, and they would have gotten their ID card years ago, before the landing records would have been destroyed. Because otherwise you have to rely on many different records form different entities scattered around to prove your identity.
UK still think to live in a past when people didn't move much, and everybody was known in their local community. The few who could move were people with many connections. That past no longer exists, just like the Empire, accept it...
Yesterday I was at a notary to put my late father's shop into liquidation. All I need to prove my identity, as the rightful heir to the notary - who I saw for the first time in my life, the shop was opened 82 years ago by my grandfather, so whoever was involved then was dead - was my ID card, no need to bring with me different documents from different entities, or look for witnesses...
Biting the hand that feeds IT
