"We are back to stability, we are open for business. There's clear political leadership," Griffith said. "The Prime Minister's set out his objectives for 2023, halving inflation, getting the economy back to growth and debt reduction over the medium term."

The sales job is a challenging one as Britain tips into recession this year, battles high inflation and a cost-of-living crisis while underperforming its European peers. Some investors have begun rethinking their support for Britain's economy after months of political turmoil, persistent Brexit uncertainties and a recession that the Bank of England said could last into 2024. Details of many of Sunak's plans to revive Britain's economy have yet to be announced.

But Griffith said the turmoil brought on by former Prime Minister Liz Truss' tax and spending plans was a one-time mistake, insisting that solid fiscal and growth plans were now taking shape.

"That was a moment in time," he said. "People understand the strength of UK institutions," he said.

In Washington, Griffith met with U.S. Deputy Treasury Secretary Wally Adeyemo, Securities and Exchange Commission chair Gary Gensler, and officials at other regulatory institutions, including the Federal Reserve and the Commodity Futures Trading Commission.