“The need to safeguard and promote the welfare of children who were in the United Kingdom was a primary consideration when immigration authorities were making a decision as to the deportation of a parent whose application for asylum in the United Kingdom had been refused. Once the children’s best interests had been identified, the authorities were then required to assess whether those interests were outweighed by any other considerations such as the need to maintain a proper and efficient system of immigration control. When the children had British citizenship that was of particular importance in assessing their best interests.”