Meanwhile, the number of companies newly listing their shares in London has dwindled.
Last year, there were 23 initial public offerings (IPOs) on the London market, with £2.1bn raised. In the US, which has much larger capital markets, there were 354 with $44bn (£33bn) raised.
It has coincided in a big rise of UK investment money flowing into US stocks in search of better returns.
“We talk as a nation about wanting growth in every postcode, but at the moment, a lot of us are funding growth in every zip code,” Dame Julia said.
There was “no shortage of great companies and no shortage of capital”, she added.
But negative sentiment about the UK market – which was often exaggerated – had contributed to companies leaving in the past, she said.
“We need to stop throwing shade at ourselves as a nation… it’s a national habit.”