Starbucks backed the offer and its more personal approach, with TV and press ads this week, while social media noise was amplified by comedian Jimmy Carr getting involved and telling his Twitter followers about it.
All of the customers we interviewed in Hammersmith were frequent Starbucks customers and none admitted to switching from another coffee chain just to take advantage of the offer.
While most took up the offer, some said they had stuck with their usual preference and had ordered something other than a latte.
One customer, Emma Friedman, said she had seen the offer advertised through Facebook. She would have bought coffee anyway, she added, but admitted she had taken advantage twice already today.
Regular customer Alex, who didn't order a latte because he doesn’t drink milk, said: "That is the busiest I've ever seen that store. There’s probably twice as many people in there as normal."
The TV and press ads were created by Abbott Mead Vickers BBDO, with media planning and buying handled by Manning Gottlieb OMD.
The offer coincided with Starbucks relaunching its latte to include two shots of espresso rather than one shot, to cater for changing British tastes.
Follow Sarah Johnson on Twitter @BRproducers