"liable to costs relating to the individual's detention and removal'' should they allow him to travel. According to the Home Office's website that charge would be £2,000"
Being a little bit cynical, but 2000 GBP seems closer to the cost of shipping him to the US than the cost of detaining him.
Even if they just denied him entry and/or deported him, it would probably would be back to either where he flew from or where his passport's from. And Hong Kong might not want him back once he's been denied entry to the UK, so to the US he would most likely go.
Being a little bit cynical, but 2000 GBP seems closer to the cost of shipping him to the US than the cost of detaining him.