"It's been over 10 years since I'm on a march," she said. "Normally we Britons stay calm and grumble, but too much is at stake with Brexit."
Armed with a homemade sign saying, "I'm British and I'm molested," Penfold said she wants the Brexit to be lifted as smoothly as possible and as quickly as possible, like many people living in London protests have been the largest since the Iraq war.
More than one million took part in the marches, organizers said. CNN has not independently audited the number.
But the question many participants of the Saturday marches asked was this: will anything make a difference?
The EU agreed to postpone Brexit until May 22 When next May's May parliament will pass May
But if the May treaty fails with MPs, the EU state and state will say Heads of Government, the UK can only stay until 12 April. Then they would expect the UK to come up with a clear plan on what to do next.
But despite the petition to repeal Article 50, May has made it clear that she believes that this would be undemocratic. And she is not in the mood for a second referendum on Brexit.
Penfold, as well as others who spoke to CNN, asked how May could arrange for Brexit negotiations with Brussels for repeated votes in Parliament, while the public received only a referendum
The marchers had many worries.
The half Portuguese Nico Hall traveled from Sheffield to London with his 4 1/2 year old rescue gundog Ginnie. He said he worries about her pet passport because he often brings her abroad.
"She's looking for a wooferendum," he said jokingly, referring to a makeshift coat he'd made out of sheets of paper that said the same thing.
The Briton Barry Englefield and his wife, his German wife Waltraud, traveled 140 miles to London from Monmouth in South Wales for the march.
"There is no we and you between Britain and the EU," Waltraud said. "There is only us all, Europeans."
She is one in three million EU citizens who do not know if they can stay after Brexit and have not applied for a residence permit after leaving the UK. "Brexit will never break us apart," Barry said.
But Penfold said the Saturday's march was about how many Britons were against leaving the block, Penfold said.
"It's about showing the rest of the world," she told CNN. "That does not make a difference in (Parliament), but I do not want everyone to know how many in the UK purely want, and we agree."