Tsvangirai left the pulpit for two minutes before delegating a question and answer session.
Some exiles asked him what the government was doing to help Zimbabweans who had been "traumatised" by violence.
"If there is anyone who has been traumatised, it is me," he answered.
Alex Chigumira, 42, who fled Zimbabwe eight years ago, said: "We can already see that Tsvangirai has adopted the politics of Mugabe.
"He is unrealistic. What he forgets is that people here are traumatised, that is why they are in Britain."
Tsvangirai was in London on his final stop of a tour of Europe and the US aimed at gaining international support for a "new" Zimbabwe.
As part of his campaign, Tsvangirai earlier met British ministers and business leaders.
Tsvangirai is expected to hold talks with Gordon Brown, the British prime minister, on Monday.