Libel or not aside, I think if a civil servant were to discriminate against the ordinary member of the public in its service in favour of an MP or Minister, the member of the public has a right to demand the same priority.
Suppose we have a hypothetical case where A, an ordinary peasant, has a private dispute, say, with his neighbour, a Minister, I am pretty sure A will be ever watchful over how the govt depts handle his affairs versus the Minister in question. If A legitimately accuses the govt dept official(s) of pandering to the whims of the powerful, he has every reason to feel aggrieved and if he can't take them to court, he can very well make a federal case out of it in the press.
