bootscooter - from a UK parliamentary answer: "the threat assessment presented by surface-to-air missiles, specifically to identifiable military aircraft, limited our use of RAF aircraft operating into Afghanistan." The point being that leased CF AN-124s would be "identifiable military aircraft" as opposed to chartered where the operator is paid danger money but it's his aircraft at hazard not DND's.
previous, similar thread relating to C-17s and Canadian Forces (as opposed to NATO):
http://www.pprune.org/forums/showthread.php?t=231644