British celebrities know what going on Celebrity Big Brother (Thursday, Channel 5, 9pm) means: it means you've slid down the celebrity alphabet, from B or C to M or N; it means you need the exposure, or the money, or both; and what it never means is a revival in your career – all it ever means is a pit stop on the long road down to the bottom.

For all of these reasons, the pool of British celebrities the producers have to choose from has been shrinking desperately – quite simply, there aren't many British actors, presenters or singers left who are willing to go on it, which is why the show has been increasingly relying on Americans (usually Americans who haven't seen the show and are more likely to say yes, or just Americans who have really bad agents).

This year, the producers of CBB have dressed up the problem of finding enough Brits as an American Versus British contest – half the celebs are Brits and half of them are American and so the series will be based on the idea that they will compete against each other.

Among the British celebrities is Gail Porter, which is worrying. The Scottish TV presenter has famously experienced a number of mental health problems including bipolar disorder, anorexia, alopecia, and post-natal depression and the first thing she said on entering the house was "I feel a little bit unwell". That's probably just a turn of phrase, but has she been getting good advice? Is someone looking after her? Is the level of care due to vulnerable people on this show being met? It may all be fine, it may not.

The other contestants are more or less a reflection of how we got here: some of them have stepped from one reality show to another; one of them met her husband on a TV contest; one did a sex tape; this show and others like it are now just rings within rings within rings. We are close to the tipping point where celebrities will outnumber non-celebrities.

So who do we have? On the American side, there is Farrah Abraham, who became known for MTV's 16 And Pregnant. She uses sentences such as "I get what I want, and until I do everyone is going to be unhappy." There is also the actor Daniel Baldwin, one of the Baldwin brothers.

On the British side there is Natasha Hamilton, one third of Atomic Kitten and the kind of woman who puts the word hashtag in front of actual words she speaks. The Loose Women presenter Sherrie Hewson is also there and James Hill from The Apprentice. There they all are, hovering round about the letter M.

So why is Celebrity Big Brother still entertaining and watchable? Why do intelligent, well-read, literate people like it? Probably because intelligent, well-read, literate people are interested in sex and Big Brother is mostly about sex. The programme allegedly started as a social experiment, with the camera as an objective observer, but now the camera is just a lech, a perv waiting for the men and women to take their clothes off, singularly or together. And it no longer even bothers to hide it.

But there is another reason we watch – the outside-lane reason. In other words, we like to slow down when something goes wrong on the road we're travelling and have a good look. There's nothing wrong with that particularly – in fact, it's perfectly normal and seeing people get things wrong helps us to get things right. That doesn't make Big Brother any kind of lesson in life but it's still interesting to press our faces up against the glass. Look, aren't they awful? Aren't they vulgar? Isn't it great that we're nothing like that?