Last Night's TV: Trinny &amp; Susannah: from Boom to Bust/Channel 4<br />Nigella Kitchen/BBC2

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As with so many momentous cultural shifts, the tipping point came with Les Dennis. There was a time when a celebrity sending himself up felt genuinely fresh; then Les, and myriad others, gratefully submitted themselves to Ricky Gervais and Stephen Merchant for Extras. They emerged looking an awful lot stranger, but also more knowing, more self-effacing, and considerably more employable.

By the time Joaquin Phoenix grew that scraggly beard, the phenomenon had become so ordinary as to be almost meaningless as an index of character or wit. If Chris Morris made his Brass Eye paedophilia special now, he wouldn't need to dupe anyone: Philippa Forrester and "Doctor" Neil Fox would never be so foolish as to turn down the chance to don a "Nonce Sense" T-shirt, lest their agents drop them like a bad habit.

Thus we come to Trinny and Susannah: from Boom to Bust, in which, as if to emphasise the point, Neil Fox is one of a number of B-listers gleefully winking at the camera while piling into the erstwhile makeover queens' attempt to relaunch their careers. With the format so established, Trinny and Susannah have a mountain to climb if they are to pull it off.

On the face of it, at least, they go in all guns blazing. They are dumped by their agent, and lose an endorsement deal with Cillit Bang. Susannah drinks constantly. Trinny undergoes a nebulous health treatment wherein a heavy-set man pummels her bare bottom. They both hate Gok Wan. When trying to shift some product at a truly depressing-looking trade fair, they resort convincingly to their most hackneyed gimmick of feeling people up to get a sense of their body shapes. "They don't muck about, they just go in for the tit," marvels Katy Wix, superb as Susannah's much put-upon assistant. "They're just... holding on to her tits."

These jokes are fine, as far as they go, and the duo launch themselves into their pantomimic selves with abandon. Surrounded by a fine supporting cast – particular highlights being Jonny Sweet's turn as the harried trade fair organiser, and Ricky Grover as a brutally matter-of-fact potential agent – they do themselves justice as actors. If anything, that's the problem. If a show like this does not feel mercilessly close to the bone, like the people involved have decided to flog their own shortcomings until their egos are a bloodied pulp, you're just left remembering that what you are watching is, after all, also quite a cunning way of getting back on the telly.

These spoofs must share one key characteristic with the reality shows they send up: the price of the spotlight is relentless exposure. You can absolutely believe in Larry David's selfishness in Curb Your Enthusiasm; and when Les Dennis anonymously phones in a celebrity spot of himself to Heat magazine, his desperation feels true. I do not believe, in contrast, that Susannah would spend the night before the filming of a crucial pilot about unemployment getting hopelessly pissed with the jobless man she is supposed to be helping to find work.

The comparison with Curb is deeply exposing. Whereas David spends a whole episode ruthlessly following the ramifications of a single instance of his pettiness, Trinny and Susannah get fired, have a dinner party, go to a trade fair, look for an agent, go to the races to give a jockey a makeover, fight with Vanessa Feltz, get commissioned to make a pilot, mess the filming up, and get miraculously hired anyway. The project was initially conceived as a series of short "webisodes", and shown online; only later were they put together for television, and the joins really show. Everything here is just too big, too generic, too Benny Hill. By going so far, the irony is that a pair who have always relied on ruthless honesty as their calling card let themselves off scot-free.

There are moments in Nigella Kitchen, Ms Lawson's sumptuous new series, when you can't help but wonder if she, too, has decided to lodge her tongue in her cheek. The food all looks delicious, of course, even if it seems improbable that anyone watching will ever try to emulate it. But sometimes the foodie doyenne is barely distinguishable from Ronni Ancona's accomplished take-off.

It wasn't always quite so lusciously mad as this. Once upon a time, Nigella would sneak downstairs to eat in her silky dressing gown; these days, she actually cooks in it. She describes a cheesecake as having "just a hint of inner thigh wibble". Urging viewers to ensure they keep a cupboard of goodies for emergencies, she idly wanders round her enormous larder, which resembles nothing so much as a very grand lady's batcave.

She's a bit like Batman in another way, too: throughout the show, she remains sumptuously alone. Friends and family come and go to eat, but their appearances feel curiously lifeless. Nigella is far more animated in glorious isolation, when you basically feel like you're watching a reclusive genius jabbering to herself. She never stops smiling, and it doesn't even look like she's doing it on purpose – she just bloody loves food. It all adds up to a peculiar, but unarguably pleasurable, viewing experience. Potty as she seems, Nigella remains wonderful: comically posh, sexily enthused, and – above all – boldly and inimitably herself.