Around 1,200 people attended the event, according to organisers.
But Davis said British legislators will be unlikely to sign up to the exit terms-which include payments of as much as 39 billion pounds ($55 billion) to the EU-without a clear trade plan.
Both the Conservatives and Labour have ruled out any second referendum.
"No-one had any idea what the consequences of Brexit were going to be".
The British parliament's only Green MP, Caroline Lucas, said: "This is too big and too important to be determined exclusively by politicians".
"This issue is far too important to to leave to the politicians".
Sir Patrick earlier told the BBC's Andrew Marr show that the "terms and conditions" of Brexit were "quite unlike" how they were presented during the run up to the 2016 referendum.
Prince Harry and Meghan Markle have chosen a wedding photographer
Miss Macaroon , Ginday's enterprise, provides training and job opportunities to disadvantaged young people . The future wife to Prince Harry appears to be crossing some to-do items off her list before the ceremony.
The EU and the United Kingdom are aiming to agree the Brexit treaty by October and to publish a framework for the future trade relationship between the two sides at the same time.
Davis is resisting this, though his warning of a parliamentary veto could come back to haunt him when legislators get to vote on the draft deal.
Both Prime Minister Theresa May's Conservatives and Labour have ruled out a second referendum after Britons voted 52-48 per cent to leave the bloc nearly two years ago, and the country is due to leave the European Union in March next year. What we are talking about is a reorientation but for me the one that comes with lots of opportunities.
The report noted that derivatives exposures of European Union banks to Britain fell from about 800 billion euros in June 2016, when Britain voted to leave the European Union, to about 500 billion euros by September previous year.
Actor Patrick Stewart, best known from Star Trek, is also backing the campaign.
Asked what he would say to people who voted for Brexit, he told the programme "what we are doing is in their benefit".
"I think we'll get a great result and we'll be able to have, not only a very big free trade deal with our friends and partners across the channel, but we'll be able to boldly go to areas we perhaps neglected over the past five years".