Yes, absolutely it used to be, although it was termed "history of depressive illness" - presumably, the thinking was that it would make you vulnerable or something. As was a history of financial problems or bankruptcy iirc- I'm going back to the 90s though.
At school, we were regaled every term with the story of a vetting visit where the person had stopped our headmistress once she said that the girl had been suspended for drinking and told her that he had no need to ask further questions as that had ruled the candidate out of consideration for the FCO. We all assumed this was a ploy to keep us in line, but in recent interviews, I've noticed they do ask a lot of questions relating to whether the individual has ever been drunk, whether you've ever seen them drunk etc. Perhaps we were all wrong 
- though tbh, I think that would leave a small pool for recruitment if previous intemperance was a bar And the QBP in my previous life was always famous for the drunken antics of the military attache...
I remember that Ulan Baator used to be the threatened posting for miscreants -where are diplomats threatened with these days if they don't the the line?
And- country advisories on the FCO website: where there's a country without UK diplomatic presence, does the info come from the closest countries where there are missions? And are countries without a presence more likely to be considered unsafe for all travel? Or is there not really much connection?
(Sorry - lots of questions!)