A day in the life of the founder of Barry's Bootcamp London, the cult gym that trains celebrities like David Beckham and Ellie Goulding

Now, it is one of many — yet it has retained its cult status.

The brand currently has three studios in Central, East and West London, and counts celebrities such as Ellie Goulding and the Beckhams as clients.

It's also a family-run business. Co-founder Sandy Macaskill, 32, runs the front of house and is also a class instructor, while his brother James covers the business side of things in the office. Both of their wives also work at the gym.

We sat down with Sandy to get a sneak peek behind the scenes at one of the most successful fitness studios in the capital.

Barry's Bootcamp was one of the first boutique group fitness studios to launch in London in 2013. It's a family-run business. Sandy, 32, runs the front of house and is also a fitness instructor, while his brother James covers the business side of things from their onsite office.

"On a day-to-day basis my brother and I are here the entire time. I teach, my bro's in the office," Sandy said.

"You get lots of investors that set studios up and then get a manger," he added. "But a manager is never going to care as much as you."

Since launching their first studio in central London, their empire has expanded by two more locations. The most recent to open was Barry's West in Queensway.

David Beckham is a known regular at Barry's London, among a host of other celebrities and athletes. Sandy has also trained Ellie Goulding and Jools Oliver.

He does still do some PT classes upon request, but it's usually for his more high profile clients, such as actors who are getting in shape for a particular role.

"To be honest, I don't think [my PT clients] end up working as hard as in one of our group classes, because you chat more," Sandy said.

Before Barry's, James had a tennis business and Sandy was a sports journalist. Anya Lahiri, James' wife, first introduced the brothers to Barry's when she was living in LA, where it was the go-to workout for actors and actresses.

"At the time we didn’t have any unisex (targeted) classes in the UK, but James and I used to go to the gym together for a couple of hours a day," Sandy said.

"Anya got him to attend the class on sufferance and he was like, 'Oh my god this is an insane workout!'"

Both James and Sandy got hooked on the workout and decided to bring it to the UK. Now, Anya is responsible for training all of Barry's London instructors.

Sandy's wife Erika is also part of the team. She handles marketing for the brand.

Sandy's typical day starts with an almond milk latte. "I got into them in Miami and I'm now obsessed," he said. If he's training in the morning he'll skip breakfast and just have some BCAAs (amino acids) mixed with water — one before he trains and one during. If he's not training until later in the day, it's scrambled or fried eggs, toast, and bacon.

Sandy teaches a Barry's class every day. On weekdays it's usually the 9.30 a.m. session at their central studio on Euston Road. After his morning class, he has a few hours to head over to Barry's West in time for his 12.30 session.

Sandy said his class schedule is designed for people to be able to train a different muscle group each day of the week. Mondays are arms and abs, Tuesdays are butt and legs, leading up to a full body workout on Friday.

If you want results, he recommends training no fewer than three times a week, but it's the clients who take five Barry's classes a week among whom he sees the most success. "Then take a few days off and maybe a yoga class," he said.

"I don't know many people who are perfect with their regime, but it's like if you're trying to learn a language, you can't just do one session a week, by Monday you've forgotten it all. Fitness is the same."

After class Sandy spends time planning his training sessions for the next day. "Every single class at Barry's is different," he said. "You'll do the same exercises but you won't know what's coming when. So if you're doing an arms class with me you know we'll do a bicep curl, but it will never be at the same point, for the same duration, or at the same angle."

"I mix up angles a lot, so from a regular bicep curl, you can turn your hands out to the sides and do a wide bicep curl, or from a squat, you can turn your feet out to make it a sumo squat."

"There are plenty of ways to tweak movements, so your body can't get used to it and plateau."

Lunch is usually on the go, occasionally accompanied by a cheeky massage from one of the team. His favourite places to grab a bite in Notting Hill are Cocotte Rotisserie, which he describes as "posh Nandos," Farmacy, or he'll grab an omelette or jacket potato from a place around the corner from the West studio.

He'll then usually sneak into trainer Louis' 4.15 p.m. class of whatever muscle group he's pushing that day, where he'll get a taste of his own medicine.

He follows the class with one of Barry's famous shakes at the Fuel Bar. "We make our own protein to make sure we have full control over our own products."

This is Barry's strategy across the board to ensure customer loyalty.

"The workout has been around since 1998 — almost 19 years. It's a very refined workout — we call it the best workout in the world because for an hour workout it really is. The strength training and cardio are the perfect combination."

So now it's a case of refining all of the other elements, such as improving the quality of the sound system, lighting, and changing facilities to keep the vibe right, he said.

The thing that most differentiates Barry's from other boutique fitness studios is the "social buzz," according to Sandy.

"We’ve created this community — there's so many ways I could sell it to people, but generally it's that. Our clients will come with a friend, or use it as an opportunity to bump into people they haven’t seen for ages. It's a very small world."

While he loves his job, he said: "It's full time — morning until night — and half of my brain is always on now that we have three studios."

So, once Sandy has finished up at the gym, he'll head home to Highgate where he'll plan music for tomorrow's classes — usually a mixture of house, hip hop, and RnB — and hang out with Erika and his 15-year old step son.

Sandy and Erika share the cooking. "I'll happily eat carbs in the evening just not really stodgy things like bread and chips, whereas I would sweet potatoes, or rice. I'm not particularly strict, it's just what feels right."

"I've got really into BBQ-ing lately," he added. "The other night I made chicken, steak and a tomato, mozzarella, and basil salad with a potato salad."

"We don’t go out that much, we're homebodies really."

"I’m about 6% body fat. If I was strict I'd probably be 4% body fat and unhealthy because I’m on my feet the entire time," he said. Of our interview, he added: "This will be the only time I sit down all day."

And he's all about balance. If he is heading out, you might find him at his favourite pub the Red Lion and Sun in Highgate Village, which is owned by Sandy's good friend Heath, who also attends Barry's, obviously.

"I don't really eat puddings, but my treat at end of the day is a glass of wine or two — I love red wine or rosé if it's nice and hot," he said.

"I don't really get drunk, but I will usually drink a few times a week," he added.

"For me, it's everything in moderation."