Context reminder: a basic free trade agreement of the kind Boris Johnson seems to have secured will reduce UK GDP by 5.2% over 15 years compared to staying in the EU, according to the government’s Office of Budget Responsibility estimate from March 2020

What if the UK uses its departure from the customs union to sign free trade agreements with other counties?

The government’s own estimate is that the biggest of these potential deals, with the US, will give just a 0.16% boost to GDP over the next 15 years https://www.bbc.co.uk/news/uk-politics-51706802

What about trade deals with Australia and New Zealand? The government estimates their effect will be around 0.00% https://www.independent.co.uk/news/uk/politics/brexit-trade-deal-new-zealand-economy-jacinda-ardern-a9571421.html