The episode Extremis has many suicide references, and one off screen but audible suicide.
Season 8+ episodes are rated higher, realising the audience is mostly long time followers (i.e. getting older) more adult approach has been taken. (As mentioned above including a few more deaths, but no where near the gore of US TV... deaths are instant & bloodless, sex is still barely, if at all there, zero nudity).
As always the doctor always prevails, meaning still that no one will loose any sleep except perhaps kids scared by irresponsible adults telling them "Dr. Who baddies are under the bed."
BBFC: 1 episode U, 111 episodes PG, 19 episodes 12.
In the UK, Series 1,2,4,6,7,8,9 and the 2008-10 specials are rated 12.
Series 3,5 and the 50th anniversary specials are rated PG.
Over all it would be rated 12: Contains moderate threat, horror, violence, scary scenes, infrequent moderate language, sex references and gory moments.
In the USA, every episode is rated TV-PG, however some should be rated TV-14.