ALISTAIR SMOUT and CLEMENT UWIRINGIYIMANA BRITAIN could send tens of thousands of asylum seekers to the East African country of Rwanda, Prime Minister Boris Johnson said, aiming to break people-smuggling networks and stem the flow of migrants across the Channel. Concerns over immigration were a big factor in the 2016 Brexit vote, and Johnson has been under pressure to deliver on his promise to "take back control" of Britain's borders. But his plan drew swift criticism from opponents of his Conservative Party and from charities. "We must ensure that the only route to asylum in the UK is a safe…