English, who has also worked at OgilvyOne, will be managing director of the and Lonergan, who has also worked at 20:20 London and M&C Saatchi, is head of client development.
Thomas will be the agency's creative director. He had been executive creative director Emea of RMG Connect, but he left the agency in October without a job to go. He had joined RMG from Saatchi & Saatchi X in December 2004.
The new agency has opened its doors with an international appointment to handle Montblanc on a pan-European basis.
English said: "Central to the Brand Advocate's offer is a planning model developed specifically for today's 'snacking' media consumption habits and the growth and complexity of new channels.
"It takes core elements from traditional brand-insight gathering, customer ownership planning and data services, and combines those with a belief in the power of ideas, a focus on nimbleness and innovation in order to engage today's customers. A flexible production platform and international focus make Brand Advocate an appealing alternative to existing agency offers."
Lonergan said: "There has been a lot of talk in the past couple of years about how customers are now in control of brands, we're here to say that brands have better opportunities than ever to build emotional bonds and that the art of storytelling is flourishing with the increase of media at our disposal.
"Our job is to advocate the brand in the dialogue between brand and customer using the right channels for the right conversation."
The group plans to launch a New York office in 2009, and other offices in Europe, South America and Asia.
Brand Advocate France is based in Paris and has been up and running for over five years.
The Brand Advocate London partners own a minority stake in the agency, with Brand Advocate France holding a majority stake and providing start-up capital.