For someone who's just got back from four days (and nights) of partying at Glastonbury, Millie Mackintosh is looking enviably fresh and energetic.

Dressed in a powder blue maxi dress that flows in the breeze as she poses for photographs in the doorway of a subterranean bar, surrounded by artfully arranged foliage and hanging terrariums, the 26-year-old looks as gorgeous and glamorous in unfiltered form as she does on her constantly updated social media feeds.

Maybe it helps that a couple of days ago, alongside boyfriend Hugo Taylor, she indulged in an IV vitamin drip - another Instagrammed moment that led to a wave of tabloid stories commenting on the unusual post-festival pick-me-up.

"They're really big in America," Mackintosh insists. "I think it's something that's coming up over here, it's going to be more familiar, people won't be so shocked by it. It seems really medical, but it's just a really good energy boost."

The former Made In Chelsea star has recently reunited with former flame Taylor, who she dated for six months during her five season stint on the hit reality TV series.

After splitting with Taylor (following a dramatic on-screen confrontation over his cheating) and leaving the show, Wiltshire-born Mackintosh began dating rapper Professor Green (real name Stephen Manderson). They married in 2013, but parted ways earlier this year.

Manderson has reportedly pointed the finger at their busy schedules for causing the rift, while Mackintosh has kept quiet about it.

And, while she's known for being open on social media, posting numerous pictures looking loved-up with Taylor, when I broach the subject today, Mackintosh makes it clear she doesn't want to discuss her relationship.

Not that she's frosty - on the contrary, she's warm and friendly, chatting openly and asking me about my holiday plans in a way interviewees rarely do.

Our primary reason for meeting, however, is her collaboration with monthly subscription beauty service Birchbox (www.birchbox.co.ut); Mackintosh has curated a collection for the July offering.

"They said, 'Do you want to do a box and choose the products that you want to put in it?', and I was like, 'Absolutely!'" she says, explaining how the venture came about.

"I love anything where I can put my own creativeness into it. Then they said, 'Do you want to do your own lip shade as well?', and I was like, 'Amazing!'"

Sipping on a rose-infused vodka cocktail and sampling the occasional canape, Mackintosh talks enthusiastically about the multiple projects she's juggling (a hair brand link-up, a potential activewear collaboration she's hoping to land), but she's most excited about her own fashion line, which in just three years has become the bestselling range on ASOS, after the site's own brand.

Unlike other celebrities, who lend their name but very little else to retail partnerships, Mackintosh is very much the boss when it comes to her brand.

"I think [other celebrities] don't necessarily own the brands, whereas I've actually founded it and own it. And I was modelling it, but now I'm using a model so that I can focus on the design and behind the scenes," she explains. "I'm more comfortable there, it's less pressure."

"Right now I'm designing next spring. I've got to finish shooting the winter lookbook, then I'll start seeing samples next week for next summer. I'm just getting used to working so far ahead, but it's going really well," she adds.

Between her own designs and her years of fashion shoots and red carpet appearances, Mackintosh's fashion credentials are second to none, so I end by asking for her top shopping tips, and am pleased to discover it's not all high-end and haute couture.

She may have made her name on chichi Chelsea, but this is a fashionista who loves the high street, and is more than happy to share her secrets...

MILLIE'S STYLE SECRETS

1. Know where to go on the high street

"You have to know where to go for certain good things. Zara do really good shoes for a night out and really nice blouses. Sandro's really good for winter coats, and The Kooples I like for tailoring."

:: Zara Fine Strappy Sandals, £39.99 (www.zara.com)

:: The Kooples Jacket with Contrasting Braid, currently reduced to £112.50 from £225 (www.thekooples.co.uk)

2. Choose a holiday look and stick with it

"I'm going to Greece for two weeks in August and I plan on wearing all white with colourful accessories, like some pom-pom sandals I've just bought"

:: Millie Mackintosh Embroidered Off Shoulder Maxi Dress, £110 (www.millie-mackintosh.com)

:: Public Desire Susanna Aztec Pom Pom Sandals, £24.99 (www.publicdesire.com)

3. Shop around online

"I don't spend much time shopping but I do a lot online. I like ASOS, Shopbop, The Outnet, a real mixture."

:: ASOS Salon Premium Floral Maxi Dress, £130 (available September; www.asos.com)

:: W118 by Walter Baker Monique Off-the-shoulder Cotton Top, £52.50 (www.theoutnet.com)

4. Embrace the athleisure trend

"I've probably got more workout outfits than I do of anything else. If I'm working out later in the day, I just run around in my workout wear all day. I've just discovered this new fitness brand that's come over from Brazil, called Live! - they've got amazing leggings and bras, and really nice prints."

:: Live! Explosion Leggins, £89 (www.liveclothing.uk)

:: Nike Flyknit Zoom Agility Women's Training Shoe, £140 (www.nike.com)

5. Comfort is key for dates

"A classic date-night outfit would be my favourite Valentino Rockstud heels, my favourite skinny jeans and a nice top or a fail-safe little black dress. Go for whatever makes you feel comfortable and confident - and that you can eat in!"

:: Dune Cliopatra Burgundy Studded T-bar Court Shoes, £80 (www.dunelondon.com)

:: ASOS Salon Extreme Airtex Midi Dress, £120 (available September; www.asos.com)

6. Have fun with festival fashion

"I like to go for different looks at a festival. I might go for something that's more rock-chick, then another day I might be more boho and feminine. It's all about mixing it up and having fun with festival fashion, because anything goes."

:: Millie Mackintosh Lace Body Suit, £45, Lace Maxi Skirt, £55 (www.millie-mackintosh.com)

:: Lipsy Love Michelle Keegan Faux Fur Gilet, £60 (www.lipsy.co.uk)

:: The 'Never-Ending Summer' July box with Millie Mackintosh is now available at www.birchbox.co.uk/subscribe (from £10 plus P&P)

GET THE LOOK

Model Portia Freeman gets our vote for best dressed at this year's Glastonbury for her borrowed-from-the-boys approach. This faded River Island jean jacket is from the men's department; pair with a cute cat purse a la Portia for your next festival outing.

:: River Island Men's Light Blue Bleach Denim Jacket, £45 (www.riverisland.com)

:: ASOS Straw Cat Clutch Bag, currently reduced to £15 from £25 (www.asos.com)

BUY IT NOW

Unlike anything else on the high street right now, Wallis' dress/trouser/top hybrid sure makes a style statement. Available in black and blush, though you'll have to be quick if you want to nab the pale pink version; it's proven extremely popular.

:: Wallis Blush Overlay Jumpsuit, £55 (www.wallis.co.uk)

FASHION FLASH

:: RETAIL REVELATIONS

Luxury fashion resale site Vestiaire Collective has mapped out the shopping habits of its users across the UK and revealed some interesting findings. Scottish women own the most Chanel bags, and Aberdeen is home to the most Chanel espadrilles, while Liverpudlian lasses have the most high heels - but trainers still outsell heels by 300% in the English city. Cambridge consumers own the most dresses (30% more on average than those in Oxford), but Dublin is home to the savviest shoppers, as its residents sell the most on the site, thereby fuelling their own shopping habits (www.vestiairecollective.com).

STYLE STEAL

Wedding season is far from over, so if you're in need of another outfit or two, the Coast sale could be just the ticket, with reception-ready frocks, skirts and jumpsuits at up to 60% off.

:: Coast Delta Bandeau Jumpsuit, currently reduced to £79 from £119 (www.coast-stores.com)