A cosy room, comfy chair, an array of drinks and snacks at arm's reach – no, this isn't a millennial's house, but their adopted office where, thanks to a combination of flexible working and not being able to afford the heating bills in their own home, they are fuelling the "flat white economy."

This sector (so-called after the less-frothy coffee beloved of hipsters) is now the largest driver of Britain's economic output, overtaking even the current manufacturing boom, according to analysis of official data by Douglas McWilliams, deputy chairman at the Centre for Economics and Business Research.

The think tank's report cited east London's Silicon Roundabout – where hi-tech companies cluster – as the hub where macchiato-swigging millennials come to roost (or is that roast?), spawning myriad digital and creative businesses.

Yes, it's easy to poke fun at their hand massaged oat "mylk" and artisanal bicycle baskets. So, so easy. But from the lavish to the ludicrous, their spending habits are helping prop up the UK. Here's how...

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Buying. Endlessly

From fast fashion to tipsy online shopping, the unlikelihood of ever being able to afford a house deposit means young people are more willing to spend on fripperies instead. Almost 60 per cent of under-37s have made a purchase while under the influence, with the average spend being (a troublingly high) £182.39.

Have the Silicon Roundabout flat white-slurpers not yet dreamt up a way to put a cap on late-night digi-purchases? Though, if these sozzled spends are halting Britain's backslide into the red, perhaps we should let them continue.

Going plant-powered

It's not just rejecting rib-eyes for cauliflower steaks: the 23-37-year-olds among us are increasingly horticulturally-inclined – you'd have to be, with most city-dwellers having only a Lilliputian 'yarden' for green space.

The UK garden market is now worth more than £5bn annually: "if we want the market to continue to grow, the entire industry needs to make sure that we help millennials to garden how they want," says Gary Philpotts, chairman of the Common Sense Gardening initiative, who added that this cohort's interest in the hobby has risen by almost a third. Boosted, no doubt, by the likes of Cindy Crawford, Kate Moss and Alexa Chung posting their plant pottering on social media. Hashtag flower power, or something.

Going the distance

For those not fixed gear bicycle-inclined, access to a shared vehicle is a must. Make no mistake, that's not a bus, but one of the on-demand cab companies such as Uber, Lyft and ViaVan, that can be summoned to your doorstep at the touch of a button.

Maybe this is another sign of the 'affordable luxury' millennials are so keen on. They go to more far-flung destinations, forking out an annual £150bn globally on tourism (last year even saw the launch of a millennials-only cruise ship), but spend three times less on accommodation than baby boomers once they get there. Proof, perhaps, that there is such a thing as being savvy and spendthrift.

Discarding plastic

Discarding as in reusing and recycling environmentally unfriendly objects, that is, not just throwing them in the regular bin like a planet-hating heathen. And it's not just bottles and bags – now payment is possible at the touch of a phone or smartwatch, credit cards are fast becoming as obsolete as cash. A good thing for cutting down on the synthetic polymers required for our 'paper' money, perhaps; less handy when your battery flatlines at the supermarket's self-checkout. How will we buy our cashew cheese now??

Hidden fur-gendas

British millennials are second only to their Greek counterparts in suffering the biggest pay squeeze in the developed world, according to a report last year from the Resolution Foundation, which found that those born around 1980 earn 13 per cent less than those who were at the same stage of life in 1970.

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And so a quandary has emerged for many: pets, or parenthood? In the United States, 75 per cent of millennials own a dog compared to half of the general population. They're not just a lot cheaper than a baby; photo-friendly pets can pull in lucrative social media, film and book deals, meaning they might actually make you money, versus the cost of caring for a human.