The car stops at the hotel and members of Tawakkol Karman's entourage hop out. It has been a frenetic morning for Karman, who became the youngest person and the first Arab woman to win the Nobel Peace Prize in December. At 33, this Yemeni mother-of-three was honoured, along with Liberia's Ellen Johnson Sirleaf and Leymah Gbowee, for her "non-violent struggle for the safety of women".
Karman issues fast instructions in Arabic to a young woman accompanying us and then she answers my question: how does she respond to those who say it is un-Islamic for women to protest in the streets?
“In Yemen, there is a culture that a woman can't go outside the house of her family or her husband," she says. "But we have led two revolutions - one against Ali Saleh's regime, and one against bad customs. These views did not come from Islam. They come from traditions, and they are wrong traditions."
In 2005, Karman set up the group Women Journalists Without Chains and began staging peaceful demonstrations for women's rights and freedom of expression. When protests swept across the Middle East last year, she emerged as a leader and is frequently described as "the mother of the revolution".
The group of officials is back in the car and we're off. Karman, in London to meet Foreign Secretary William Hague and give several talks, is in a hurry, but we have taken a detour to the hotel to fetch one of her trademark colourful hijabs. She takes it from her companion, smiling, and continues talking as she adjusts the floral headscarf she is wearing.
“We started in 2005 because we believe that women must struggle for all the rights of the society, not just their own. There is no citizenship in Yemen - for men or women. Citizenship is only for the dictator and his family."
Somehow, with deft hand movements, she has changed her hijab without exposing a single hair. Most women in Yemen wear the niqab, a full face-covering, usually black, but since 2004 Karman has covered only her hair, making the point that the niqab is cultural, not religious.
Last month, Ali Abdullah Saleh stood down as president after 33 years in power. Yet Karman is scathing about the Gulf Co-operation Council Initiative, which led to Yemen's first elections in decades. "We don't recognise this initiative. It is against human rights, against democracy, against all the demands of tens of thousands." Her contempt is understandable: the agreement dictated that the only candidate be Abd-Rabbuh Mansur Hadi, Saleh's vice-president. It also grants immunity from prosecution to Saleh and all who worked with him.
The main aim of Karman's visit to the UK is to lobby the west to support the revolution by freezing Saleh's assets and investigating him for crimes against humanity. I wonder why western countries have been slow to act in Yemen. "You have to ask them this question. I don't want to speak about the past. I am now talking about the future."
With her tiny build, Karman looks far younger than her 33 years. She is forceful and impassioned when speaking, and I can see why some describe her as aggressive. I try to engage her on the subject of al-Qaeda in Yemen: the perception of the country as a ‘terrorist hotbed’ is undoubtedly one reason that the US and Europe were reluctant to pressurise Saleh. She will not be drawn: “I think the international community has stopped talking about that because they know Ali Saleh was supporting terrorists.” A level of impatience with the western obsession with al-Qaeda is fair – security has been used as an excuse for human rights violations in Yemen for years – but unfortunately, it cannot be written off.
Later, I speak to Kate Nevens, a Yemen expert at Chatham House. She explains that before the Nobel Prize there was some scepticism about Karman's role. "She's definitely not seen as a representative of everybody, but the response to the prize was resoundingly positive."
Although she is a member of al-Islah, part of the formal opposition, Karman identifies closely with the revolutionary youth. As we drive down the Embankment, I ask her what impact the prize had. "When I was in my tent, we felt alone," she says. "But now we know our voice reached the international community."
Karman has been arrested many times and faced several serious threats to her life. Does she worry about the danger? "I have never asked myself this question. Since before the revolution, my life was threatened. But what is the situation of people in Yemen? They are in danger. The question is how to make Yemen peaceful."
Her translator - whose skills have gone largely unused - points to the London Eye through the window. Karman stops talking about the threats to her life and leans across me to look out at it, gasping and smiling in delight.
It is easy to see that she is a natural leader. Charismatic and ready to smile, she is also formidably resolute. Earlier at her parliamentary briefing, she was asked whether women would revert to being oppressed after the revolution. “Do you think a woman like me will go back?” she responded, to barnstorming applause.
But it is a serious question, which I follow up on. “People shouldn’t ask whether a woman will go back to her role,” she says. “They should ask how we can work together to make women safe and to protect their leadership.”
At times, I wonder if Karman is overly optimistic. Yemen is beset with tribal conflicts and Saleh has entrenched the influence of his family and associates. "Stepping down the dictator is just the first goal, it isn't the end," she says. "The revolution is something continuous. We win when we achieve all our goals - reaching democracy, creating a constitution that guarantees human rights."
Finally, I ask whether she still believes that Yemen will become a democracy. "We will succeed," she insists. "You can't make the future by just seeing the present. We have to be part of changing the future."