I used to assume that someone was employed at the BBC to vet all recorded output that was repeated. Maybe that person went when the Beeb started cutting costs, and maybe now that position will be restored?
On the other hand, I can recall stuff falling through the net in the 80s, like a Laurel & Hardy episode in which they are in blackface. The older the material, the more likely to offend, but it tends to be that old stuff that just gets showed into the schedule unvetted.
Guidelines exist for producers in the first instance of course, and, as one might expect, are pretty stringent as regards kids' shows. However - if
Tweenies for example was 'passed' back in 2001, it's highly unlikely that anyone at the top would've considered needing to reappraise every single episode in the time since. Attitudes haven't changed
that much in the past dozen years. (Unlike the way that they have since Laurel & Hardy's films were produced...)
Still, this was one very rare instance in which an episode
should have been double-checked, obviously, and highlights my earlier point about the mass-production and wall-to-wall nature of kids' channels per se...