Trump has nominated Arkansas businessman Warren Stephens to be his ambassador in the UK.
The UK government is currently in the process of picking its US ambassador, with former Labour minister Lord Mandelson believed to be a contender for the role.
In his speech, the PM repeated his promise to rebuild and renew ties with Europe, which he said were vital to Britain’s security and prosperity.
He insisted it is "deeply in our self-interest" to support Ukraine against Russian president Vladimir Putin because "the future of freedom in Europe is being decided today".
The UK is now "determined to fight harder on the world stage for our national interests and ready to dig deeper to defend them", he said, because a win for Putin would damage "our own security, stability and prosperity".
"So we must continue to back Ukraine and do what it takes to support their self-defence for as long as it takes.
"To put Ukraine in the strongest possible position for negotiations so they can secure a just and lasting peace on their terms that guarantees their security, independence, and right to choose their own future."
Supporting allies was what former prime ministers Clement Attlee and Winston Churchill had done, Sir Keir said, adding that he thought of the Attlee government of 1945 and its ambition to build "a country fit for heroes".
"And they saw that maintaining our strength abroad gave us the foundation to succeed at home. That is as true today as it was then," he said.
Now was the time, Sir Keir said, to "strengthen our security as the bedrock on which the economy rests – and the ultimate guarantor of everything we hold dear".
Such words may be tested come January if the US imposes tariffs on European goods and demands the continent trades less with China, all while forcing Ukraine to cede territory.
Conservative Party co-chair Nigel Huddleston said it Sir Keir's government had "set our country back" since Labour took power five months ago.
He said: "From driving business confidence to near-record lows, working people punished with a jobs tax, growth projections slashed, and a dash to surrender British interests overseas - it’s no wonder he’s been forced into a desperate reset."