I completely agree with what you say, Guy. The civil service must remain independent and impartial. There are certainly people who, for their own ends, want to politicise it and those people must be resisted. It is a matter of judgement, however, how far it has been politicised? As one example, it is possible that the documents were leaked by another civil servant who has some sort of beef with Darroch. If that is the case then it is not really evidence that the civil service has been politicised nor could it be viewed as an attempt to politicise it. If, however, a politician from the leave side leaked the documents in order to get Darroch out of post, because he has remain views, then it would very much be evidence of an attempt, successful in this case, to politicise the civil service. At the moment we don't know.
Reports from diplomats to London come in many forms with different levels of classification. Some have very limited circulation, e.g. Prime Minister, Foreign Secretary and Defence Secretary only. Some have wider circulation perhaps going to ten or a few tens of MPs and officials. Some go to hundreds of people. I am not sure who decides what level to allocate to each document or on what basis. Does the sender allocate a level to it? Does someone in FCO do it? I have no idea but it seems likely that the leaked documents may well have been on the hundreds circulation list. Given the language used in them perhaps whoever decided should rethink their criteria.