The problem with using military aircraft in this situation is permission. Libya (or any where else en route) may, probably will, give permission for a civil aircraft but a military aircraft has very different connotations (even if only a passenger machine) and will probably be refused, particularly with a regime like the one we see in Libya.
That's not to say that they can't be used, just that if we did use one the whole risk assessment both for that flight and subsequent civil flights would be very, very different.