What do they call them in the UK?
'Street seller' and 'Street vendor' are fairly neutral terms, but two words rather than a single word.
'Hawker' and 'Peddler' / 'Pedlar' (Br.E.) are also used, but might have a more negative connotation (I'm not 100% sure, I'm not a native speaker). They're also used for people selling other kind of goods than food, and from e.g. a blanket rather than a cart.
one who offers merchandise (such as fresh produce) for sale along the street or from door to door
In this specific case of a coffee stall we could say coffee vendor to describe the Barista.
The general phrase for seller of food and beverages and other mechandise is Street Trader
Street Traders that serve hot food or drink at any time between the hours of 23.00 and 05.00 will also require a Premises Licence
Peddlers are permitted either door to door or on the streets with a low cost certificate after they have lived in the area for 28 days, however their means of conveyance must be no more than a small trolley. If you work as a pedlar without a certificate - or let someone else use yours - you could be fined up to £200. (However you have to catch them first).
Fixed pitch licensing ranges from the cost of a peddlers certificate all the way up to £#,000's depending on locality, square area, spirits and duration. With a stall as shown we commonly use the single word Stallholder http://www.thamefoodfestival.co.uk/stallholder-selection-process/
Definition of stallholder
: one who manages a stall at which articles are sold
I disagree with the Cambridge definition of "a person who rents or owns a stall in a market"
Over the years markets halls have reduced and stalls are more commonly found in the street unless at a historic weekly market.
They are sometimes manned by an employee or franchise holder, but collectively holder is used when referring to a non specific group of people who hold a fixed pitch with a permanent licence.
"To fund his education he rented a stall at Portobello Market, bought bits and pieces from other stallholders, then sold them to the public at a healthy mark-up."
Some marketeers are now highly respectable think
Sir Richard Branson or Sir Alan Sugar
Interesting aside about this market stall the clock repair stall run by Mr Michael, whose well-known customers have included Richard Branson, Margaret Thatcher, Princess Diana and the Queen. https://www.independent.co.uk/news/celebrated-fashion-market-is-forced-to-close-after-30-years-1119888.html
The more traditional single word for a street seller is costermonger "... has come to be a synonym for street vendor"