You can´t! Arriving illegally? Then they stay without permission, and you are a citizen. That was the first advantage. You are legal and they are illegal.
After that, can they as an illegal alien open a bank account in UK? But you can, right? Can they get a registered work, pay tax and get a pension? Answer no! You can, YES!
Can they even get a driver license as an illegal alien? No I don´t think so. And the obvious list goes on.