Last Night's TV: The Untold Invasion of Britain, Channel 4<br />Big Meets Bigger, BBC3<btr />Gareth Malone Goes to Glyndebourne, BBC2

The Untold Invasion of Britain is a terrific idea. In fact, so is the whole of the Bloody Foreigners series. Each episode looks at a separate point in our history when people from abroad have played a pivotal role. It's a neat way to dispel any little islandism, and an enjoyable learning curve to boot. It's just a shame it has been so hammily done. Simply told, the story would have been interesting enough. Septimius Severus, the Libyan leader of a Roman military division, marches to Rome to seize power from the traitorous Praetorian Guard after their assassination of the incumbent emperor, then decides to expand north of Hadrian's Wall. Once there, he finds a population of surprisingly civilised savages putting up a jolly good fight against the mighty Romans. Well, who could resist that? Severus even had to contend with familial treachery: not only did his son disobey him but – just for good measure – tried to stab him, too. This, surely, is a soap-writer's idea of heaven.

And yet, and yet... Channel 4 appeared determined to ruin it all with cringe-inducing reconstructions and bizarre graphics. Inexplicably, each snippet – which, properly done, could probably have enhanced rather than detracted from the overall result – was wrapped up in a fluff of CGI, slow-motion and sepia. Even the dialogue had reverb on it. It was unclear why: were they hoping to give the impression that Severus was speaking down the ages? That we were catching sight of him through some kind of time-travelling mechanism that left the speed of sound and light lagging behind?

At any rate, it wasn't, suffice to say, the best of effects. The acting wasn't great either, though I very much doubt the cast can be blamed, given the colourless parallel universe that they had been asked to inhabit.

It's a shame, because the bits where we got to focus on the plain old history were great. Historians and archaeologists were queuing up with interesting titbits. We got to see the site of a northerly Roman battle camp – all 170 acres of it – and the South Shields site of an old food-distribution unit. From the looks of things, old Severus brought leagues of Libyan troops, too: you can still see their stone roundhouses. It's remarkable, really, to think of that: Scotland's first African immigrants, almost 1,800 years ago. Hopefully, we'll get another chance to hear about it; this time without all the FX trimmings.

BBC3 has given us something like Big Meets Bigger before. My Big Decision, I think it was called, almost a year ago. It's a weirdly Channel 4 kind of idea: two young women, both overweight, get sent to Mississippi to see what might happen if they expand too much further. Bex and Ann were our two candidates; Bex, who drinks for confidence and desperately wants a boyfriend, and Ann, who likes cheesy pasta and whose mother has diabetes.

Anyway, once they got to Mississippi they met Dolores and her family, who make them look positively emaciated by comparison. Bex and Ann are overweight, yes, but not cripplingly so. They are both still pretty, active and able. Dolores is huge: 38 stone. She cooks huge meals of soul food, plates of pigs' ears and back fat, and goes to her local buffet (where there are more than 100 dishes) several times a week. She hasn't walked as far as her mailbox for years, and can't get dressed unassisted. She has all kind of health problems: diabetes, high blood pressure, chronically sore feet. It was dreadfully sad, especially when the inevitable box of photos came out. Both Bex and Ann had been big since youth; Dolores, on the other hand, piled on the weight later in life. It was a response to grief, she claimed.

Naturally, Bex and Ann claimed it had altered their perspectives on life, and swore to make changes to improve their health. Ann even started jogging, which allowed her to lose 16 pounds. Of course, it was fairly standard stuff; much of the programme smacked of little more than the ubiquitous fat porn that we've been bombarded with recently. There was, however, one redeeming quality: the fact that everyone – Ann, Bex, Dolorers, Dolores's family – we met was absolutely lovely. Really, upliftingly, delightfully lovely. And that, in itself, was a pleasure to watch.

It was never really going to be a disaster, was it? The whole thing was set up as a triumph of opera over adversity. And, sure enough, Gareth Malone Goes to Glyndebourne ended pretty much as expected, which is to say, with applause, kudos and a sentimental sense of success. Malone has done a great job, it has to be said. Plenty of the teenagers had seemed positively tone-deaf on first encounter, not to mention wholly uninterested in the task at hand. And he hadn't, once, strayed from his likeably self-deprecating tough-but-fair persona.

Things couldn't be too straightforward, of course, so just as the kids looked like they might be getting there, Gareth decided to introduce a "mothers' chorus" for an element of will-they-won't-they-pull-it-off appeal. They did, in the event – pull it off, that is – though I do rather wish it had just been the kids on stage and the parents in the audience.

Anyway, it was an excellent series – against all expectations: it had all the chance in the world to be an excruciating case of the middle-classes-giving-the-hoi-polloi-a-lesson-in-culture; but it wasn't. It was great. More please, Gareth.