The Prime Minister must make sure that Britain isn't worse off economically as a result of leaving the Single Market and the Customs Union at the end of this year. Up till now 46% of all our exports (worth £289 billion per year) have been to the European Union. Our best-known local MP must therefore ensure that by leaving the European single market and the customs union at the end of this year, we don't lose those exports (and the millions of British jobs that depend on them). In 2016 he promised that Brexit would not economically disadvantage the UK.. 2021 will begin to reveal whether he can deliver on that crucial promise. He must ensure that the implementation of Brexit and his post-Brexit trade and other policies do not harm any sector of the U.K.'s economy.

Brexit-related economic promises from Boris Johnson and his colleagues:

On 6 March 2016 in the run-up to the referendum, Boris Johnson encouraged voters to believe that "the cost of getting out [of the EU] would be virtually nil"
Then just two weeks before the referendum, the official Brexiteer campaign organisation, Vote Leave (whose most famous campaign committee member was Boris Johnson) issued an official statement, promising that “after we vote Leave, we would immediately be able to start negotiating new trade deals with emerging economies and the world’s biggest economies (the US, China and Japan, as well as Canada, Australia, South Korea, New Zealand and so on), which could enter into force immediately after the UK leaves the EU”.

They also claimed that Britain would still 'have access to the Single Market after we vote Leave'. “Absolutely nobody is talking about threatening our place in the single market,” said Tory politician, Daniel Hannan, a founder member of Vote Leave .
Then on 26 June 2016, just three days after the referendum, Boris Johnson wrote an article in the Sunday Telegraph promising that "there will continue to be free trade [with the EU] and access to the single market" and that “British people will still be able to go and work in the EU".

Then, on 3 November 2016 (some five months after the referendum) Johnson promised the public that “Brexit means Brexit and we’re going to make a titanic success of it”. Lets hope that particular maritime reference wasn't too appropriate!
Then on 11 July 2017, he promised Parliament and the public that "There is no plan for 'no deal', because we’re going to get a great deal".

And less than a year ago, in his New Year's prime ministerial message, delivered to the nation on 31 December, 2019, he sought to persuade the public that post-Brexit Britain will be a country "of growth, prosperity and opportunity".

Many top Brexiteers wrongly but sincerely believed that they would be able to force the EU to give the British government whatever it demanded. Michael Gove, joint leader of the 2016 Vote Leave campaign (and now a senior member of Boris Johnson's government) said just six weeks before the referendum that “The day after we vote to Leave, we hold all the cards - and we can choose the path we want".

And some other senior Brexiteers deluded themselves that the world at large would give the UK government virtually whatever it wanted. Just after their referendum victory, David Davis (then Tory Secretary of State for Exiting the European Union) sought to persuade the British public that "Within two years" the British government will be able to "negotiate a free trade area massively larger than the EU" and that those "new trade agreements will come into force" in "fully negotiated" form as soon as Brexit happens. “Be under no doubt: We can do deals with our trading partners, and we can do them quickly." the senior Brexiteer Tory told his party.

Davis and his Brexiteer colleagues were so naively confident of his government's influence in the world that on 10 October, 2016 (just four months after their referendum victory), he told the British public that "There will be no downside to Brexit, only a considerable upside".