Part of the armoury of a successful comedian is a catchphrase, individually tailored. But newscasters have them too, although these are shared among the whole profession: "images that some may find distressing", "look away now" and – our text for today – "this report may contain some flash photography".

The latter phrase frequently bemuses foreign viewers watching British TV. The obsession with alerting the audience to the equipment of the paparazzi is also the subject of frequent baffled queries on TV blogs.

The explanation for this admonitory sentence turns out to be a little parable of the continuing power of television regulation in Britain. Questioned on why Huw Edwards and Fiona Bruce spend so much energy anticipating flash-bulbs, the BBC seemed oddly reluctant to answer, but referred me to Ofcom, who revealed the flash alerts are a regulatory requirement because of the risk to viewers with epilepsy.

The charity Epilepsy Action helpfully explained the background. About 5% of people with epilepsy in the UK have been diagnosed with the photo-sensitive variety of the condition, which can be triggered by fast flickering. Images only become dangerous at 25Hz, but, given the difficulty of testing every piece of film in advance, the solution is for broadcasters to issue general warnings.

What's striking – and oddly touching – about this is that there are only an estimated 2,250 photo-sensitive people with epilepsy in Britain, all of whom are unlikely to be tuned into the news at any one time. And so a bulletin aimed at millions of viewers contains one sentence of script relevant to a handful.

And, paradoxically, the news items intended to be of most general interest – George Clooney at a movie premiere, a scandal-hit celebrity leaving a hotel in a storm of paparazzi lightning – are also those of most concern, for a different reason, to this niche sub-section of the audience. Ofcom couldn't stop ITV axing The South Bank Show but it can still reduce the risk of epileptic seizures. These are the parameters of television regulation.