“…ower to detain be exercised reasonably and for the prescribed purpose of facilitating deportation. As Lord Justice Carnwath said in R (Dritain Krasniqi) v SSHD [2011] EWCA Civ 1549 at [12], they are no more than applications of two elementary propositions of English law: first, that compulsory detention must be properly justified, and, se…”