It is no longer a secluded medical refuge for royals, aristocrats and celebrities who can afford its hefty price tag. Rather, you are just as likely to spot a builder being treated for a bad back as you are an oligarch's wife waiting for plumper lips.
"People do have misconceptions about Harley Street," says Engle, 52. "For most people it is synonymous with private medicine, while for a younger generation it is synonymous with cosmetic surgery.
"But people go to Harley Street all the time - I know people who have gone, I might have been treated there as a child. That is the thing about that place; everyone knows someone who has been. It is not just for celebrities and royals.
"There is one clinic that treats a lot of builders with bad backs who cannot work otherwise.
"I like that story because people hear 'builders' and they do not think 'Harley Street'."
Distastefully snobbish, perhaps, but true. Its reputation for providing top-notch medicine to the upper-classes has held ever since the best in the business (including Florence Nightingale) flocked to Harley Street to treat local wealthy patients in the nineteenth-century. In 1860, there were just 20 doctors on Harley Street; by 1914 there were 200 and today there are in excess of 2,000.
Like their predecessors, medics still seek out the W1 postcode. In the series, Engle follows estate agent Jeremy Cohen around the block while he tells her that many doctors - some disillusioned with NHS bureaucracy - are more than willing to pay £100,000 a year in rent before they have treated their first patient.
"Harley Street is definitely a business. It is about that big brand name, it is about marketing" says Engle. "If a doctor has an office on Harley Street, his name will come up higher on a Google search list - and that is worth a lot for businesses."
She continues: "This is also about the cost of treatment. Some woman who will pay £10,000 for a health check, and that raises financial questions, questions about people's lives, their values and priorities."
I assume they are paying for the best medicine available in the country? "I think that is a big misconception," responds Engle, sharply. "All the doctors who trained in this country were trained by the National Health Service. Most of the doctors who work on Harley Street still practice in the NHS.
"By and large, you still find them in both places.
"People are paying privately for a consultant of their choice at a time of their choice. If you go to Harley Street, you are getting convenience."
And then I sense that she starts to get defensive - quick to distance the series from any healthcare policies that might be raised during the election campaign: "This is not about private versus public healthcare. There is nothing polemical about this programme.
"The vast majority of us use the NHS exclusively, so this is a window into a world that most people will not experience.
"I have no interest in bashing the NHS."
So, I ask, why interview doctors who say they are disillusioned with the NHS? "I think there are a lot of practitioners who are disillusioned with the bureaucracy of the NHS, but they are wholly committed doctors who are also committed to the NHS.
"I can easily believe, working for a public sector organisation myself, that bureaucracy can grind you down."
To date, Engle has made more than 50 films for the BBC over the past 27 years, including The Jews, Lefties and Walking With Dogs. But, she says, she never walks into a project with an idea of what she will find.
On this point, she is firm.
"If you do my job, you can never go in and guess what you are going to find. Experience tells me that life is not like that," she says.
"The whole reason I am a documentary maker, the reason I do not make fiction films, is because truth is stranger than fiction. Real people are far more beguiling than anything you can make up.
"I try to be the voice of the viewer. I try and ask the questions they want asked. The reason I include my questions in my films, is honesty. If someone being interviewed is surprised, defensive or amused, by including my questions the viewer understands why."
Engle is renowned in the industry for getting access to shut off communities.
She believes that her reputation has helped her gain access, but admits: "There are no short cuts.
"It is very hard and takes a long time. You have to ask a lot of people if they will be interviewed, then they want to know what your intentions are, and then they want to go away and think about it. It is a very long process.
"People go to Harley Street for privacy and discretion, so you cannot just turn up and start filming.
"You have to kiss a lot of frogs before you will find anyone who will agree.
"It is a very long and complicated puzzle that you are putting together, so you just have to be extremely patient and respectful."
She adds: "I wanted to go to Harley Street because there is this whole different world there. It is a world that conjures up all sorts of issues and broad questions about life, death, aging, and family - the whole nine yards.
"From the very earliest bits of research and ringing names on doorbells, it was apparent that there was so much going on there. You have an idea, develop the idea and discover the riches.
"We found that there isn't a big community in Harley Street because doctors and surgeons turn up at 7am, disappear into an operating theatre and then go home at 9pm. They are not sitting in a bar chatting."
So what did she learn from her series? "I cannot really answer that. I knitted together pieces of a jigsaw puzzle and found a whole raft of ideas. There is not one thing I could take from it. You keep discovering new things and question things you might have already have had ideas on such the NHS or private medicine."
North London-based Engle, who is now working on a documentary about domestic violence, says her Jewishness is a key part of her character.
"I would call myself a secular Jew," she says. "What does that mean? I suppose I am culturally very Jewish.
"I like to think that I am warm, humorous, I definitely eat a lot and I feed people quite a lot of food. I am an anxious person; there are many things about me that I associate with my Jewishness. I am definitely very Jewish."
