The BBFC originally proposed that 12A certification would have a hard floor at 8, but a variety of industry interests stopped it. I find it moderately outrageous that many parents now treat 12A as a synonym for PG which is in turn a synonym for U, which results in (to quote a real life example) a petrified child sat with their chav mother at the opening night of The Dark Knight (a film which was, by pretty general consensus, under-certificated at 12A and should have been a 15).
But on the other hand, TV programmes have bizarre certification issues. To take Doctor Who, it's shown at 6.45 or 7.15 on a Saturday, so can be assumed to be as mainstream family entertainment as it gets. On the other hand, the second and fourth post-reboot series are certificated 12 as DVD releases, although the third is PG. Something's messed up there. Meanwhile Torchwood, which veers wildly in tone, has been probably accurately certified at 15 but is marketed as just another DW spin-off. But Don't Look Now is now certified 15, as is The Wicker Man, both of which have many more issues for children (they were originally released as a double bill: what a night at the flicks that must have been).
One problem is that a whole set of films that parent today will remember from their youth are ludicrously over-certified. Both Bladerunner and Once Upon a Time in the West have 15 certificates, as they were originally AA (14) and all AA films were bulk-changed to 15s (at the time, the only alternative would have been PG: 12A came later). The former is clearly less troublesome than The Dark Knight, the latter is for practical purposes just another Western (albeit the best ever made). As a result, even well meaning and engaged parents may assume that 15 now is OK for children, on the grounds that films they know are 15s; unfortunately, today the BBFC will certify pretty well anything that doesn't contain graphic sexualised violence at 15. Don't Look Now, The Wicker Man and Apocalypse Now are now 15s because they were Xs, then 18s, and have been re-certificated as part of restoration and re-editing. But then, both Son of Rambow and Be Kind Rewind, which my daughters loved at about ten, are 12As (they saw trailers, they wanted to go).
It's all a shambles. I don't like the conflation of sex with violence, I don't like the marketing of adult films to children; I had a heated correspondence with the BBFC over trailers for Juno being shown in a screening of a U film, and I really object to PG'd trailers for 18 films being shown in early evening 12A presentations. I think that in cinemas, no trailers should be permitted for any film whose certificate is higher than the feature presentation. I don't like the way the BBC constantly ups the ante, by using Sarah Jane Adventures to draw in an audience for Torchwood as just another spin-off. I don't like the way that parents insist that their children are somehow older and encourage them to consume older material (and, in passing, take their children to workshops and courses that are too old for them because they are just so advanced). I've decided to stick absolutely to certification in cinemas (ie, you go to 15s when you are 15 and not a day before), pre-screen by seeing them myself 12As unless the Parents' BBFC site (which I heartily recommend) seems unambiguous and only permit DVDs under supervision (we only have one TV).
But the kids love Doctor Who. And have done since the Ecclestone off, when they were about 5 and 7.