"It's been over 10 years since I've been on the march," she said. "Normally we stay Brits quiet and clouded. But with Brexit there is too much at stake."
Armed with a homemade sign that said, "I am British and I'm bothered." Penfold said she wanted Brexit to be smuggled as smoothly as possible and as soon as possible as many of the people who attended London's greatest protests since the war in Iraq.
More than a million took part in the marches, the organizers said. CNN has not independently verified the figure.
But the question that many participants of Saturday's marches asked themselves was this: Will it do anything?
The EU agreed to delay Brexit until May 22 if May gets her repurchase agreement passed by Parliament next week.
But if May's deal fails with MPs – again – EU leaders said Britain could only stay until April 12, then they would expect Britain to submit a clear plan on what it would do next walk.
However, despite the petition to revoke Article 50, May has made it clear that she believes it would be undemocratic. And she has no mood for another referendum on Brexit.
Penfold, as other marchers who spoke to CNN, questioned how May could bring her Brexit dealt with Brussels for repeated votes for Parliament, while the public only got a referendum to get their voices heard.
The marches had lots of worries.
Half-Portuguese Nico Hall traveled to London from Sheffield with his 4 1/2-year-old rescue dog Ginnie. He said he is worried about his pet passport because he takes her a lot abroad.
"She's after a wooferendum," he jokingly said, referring to a deliberate coat he had formed for her out of sheets that said the same thing.
Brit Barry Englefield and wife his German wife Waltraud traveled 140 miles to London from Monmouth, South Wales to March.
"There is no one and us between Britain and the EU," said Waltraud. "There are only us all, Europeans."
She is one of the 3 million EU citizens who do not know if they can remain after Brexit and have not yet applied for permission to remain after the UK leaves the block. "Brexit will never break us apart," Barry said.
But Penfold said Saturday's march was about showing how many Britons opposed leaving the block, Penfold said.
"It's about showing the rest of the world," she told CNN. "This can't be a difference in (Parliament), but I want everyone to know how many in Britain want, not out. And we agree."