It's not as if Britain has no experience of dealing with illegal immigrants and economic migrants. Hong Kong under British rule kept them in secure detention centres before processing and repatriation. For Chinese illegal immigrants from the Mainland it was the next day. For the Vietnamese the HK Govt footed the bill until their refugee status was decided; whereupon they were either shipped back to Vietnam or allowed to settle in HK or another foreign country. I'm not sure if the UNHCR ever settled the bill.