[Reuters Analysis] 'Cool head' approach defines UK Starmer's balancing act over Ukraine

• Keep cool heads is Starmer's message to UK government
• Hopes to be a 'bridge' between Europe and the U.S.
• Balancing act may get harder

LONDON, March 5 (Reuters) - "Cool heads" is the mantra of British Prime Minister Keir Starmer as he tries to heal a rapidly growing rift between the United States, Europe and Ukraine.

Starmer, who has played up his role as go-between over Ukraine, faces a diplomatic balancing act to keep both Europe and U.S. President Donald Trump on side and protect Britain from U.S. tariffs that would damage his country's strained finances.

He has formed an unexpected alliance with French President Emmanuel Macron, a sharp critic of Britain's departure from the European Union, and a solid relationship with Trump, who UK officials say likes Starmer's lack of pretension.

"You are a very tough negotiator ... I'm not sure I like that, but okay," Trump told Starmer at a press conference after they met at the White House on Thursday. Starmer had earned his salary with his efforts to end the threat of U.S. tariffs, Trump said, without saying that he had succeeded.

Since Trump upended Washington's supportive approach to Ukraine earlier this year, Starmer has helped to defend Kyiv's position while not offending the U.S. leader, who wants a swift peace deal, with or without President Volodymyr Zelenskiy.

After convening talks with European leaders in London on Sunday, Starmer said Britain, France and others would work on a proposed peace deal to deliver to Trump and "a coalition of the willing" to defend it.

He and Macron hope their leadership can help them rescue their reputations at home and cement European security as Germany recovers from months of domestic political uncertainty.

But Starmer's announcement did not say which other nations would offer peacekeepers or step up weapons deliveries quickly.

Some countries have reservations about sending troops - Poland, which a person familiar with national security said was coming under "significant pressure" to send its forces to Ukraine, says no decision will be made before elections in May.

Britain is still hopeful of getting others on board, but time is of the essence, with Trump ramping up the pressure by pausing the delivery of military support to Ukraine.

Starmer urged nations on Sunday "to keep the pace behind these actions", adding "this is not a moment for more talk – It is time to act".

'TIME FOR COOL HEADS'

A trained lawyer sometimes criticised for a lack of humour, Starmer's dispassionate and methodical approach at the White House talks and subsequent meetings in London with Zelenskiy and European leaders at the weekend, has been widely praised.

"This weekend he has not really put a foot wrong," former foreign minister James Cleverly from the opposition Conservative Party told parliament on Monday.

While Starmer has yet to win U.S. security guarantees for Ukraine, he has worked to repair the unprecedented rift in U.S.-Ukraine relations encapsulated by the heated exchange between Zelenskiy and Trump in the Oval office last Friday.

A British source said Starmer has been advising Zelenskiy on what would close the gap with Trump. The Ukrainian leader wrote to Trump on Tuesday saying he was ready to negotiate. Starmer called Zelenskiy earlier that day, praising his commitment to securing peace.

It was the surprise phone call between Trump and Russian President Vladimir Putin on February 12 that spurred Starmer into action. "Time for cool heads," said a government source at the time, using a phrase that officials repeat.

That prompted a meeting with Trump's special envoy to the United Kingdom, Mark Burnett, a day later when the U.S. president called in - the first of five phone conversations and one meeting with the U.S. leader since.

Those conversations have been interspersed with calls with Zelenskiy and Macron, a relationship described by one British official as being presented to Trump as so close he couldn't get 'a cigarette paper' between them".

'PRINCE OF DARKNESS'

Starmer's team said the British leader had made some progress with Trump, citing the U.S. leader reiterating Washington's commitment to NATO's Article 5, which states that if a member country is attacked, all members will consider it an attack on themselves.

Despite the setbacks, Starmer is ploughing on with drafting a peace plan, while his newly appointed ambassador to the U.S., veteran Labour troubleshooter Peter Mandelson, keeps the Trump administration on side.

Mandelson, dubbed the Prince of Darkness by the British media for his negotiating skills behind the scenes, suggested on U.S. television on Sunday that Zelenskiy back the mineral deal with the United States and get "back on the same page" as Trump.

Starmer is betting that if European countries can come back with pledges to increase defence spending and a plan to ensure the security of Ukraine, Trump will offer U.S. security backup, something he says is vital to deter Putin from attacking again.

Tobias Ellwood, a Conservative former lawmaker who has chaired the parliamentary committee on defence, said it was important to get the balance right and prepare to work without Washington if necessary.

"In the same way that Trump respects bullies who stand up to him, we must stand firm in what we believe in," he told Reuters.

Addressing top ministers on Tuesday, Starmer said of his seven days of shuttle diplomacy that "everyone was aligned in pursuit of a plan that will bring lasting and durable peace to Ukraine".

He told ministers, according to his spokesperson, "he would continue to have conversations at pace".