British Prime Minister Keir Starmer, who on Monday announced he would resign, may not have been that popular with his British constituents lately, but he certainly had many friends among the European Union's top politicians.

As European Commission President Ursula von der Leyen reacted to his resignation on Monday, she wrote: "European and Ukrainian security is stronger because of you. Thank you, dear Keir."

French President Emmanuel Macron also expressed gratitude for Starmer's involvement in Europe's "coalition of the willing" for Ukraine, and for revitalizing EU-UK relations.

It was as a supporter of Ukraine in its war against Russia that Starmer really cemented his reputation in Europe. Together with Macron, Starmer was a leader of the so-called "coalition of the willing." The alliance of 35 states supports Ukraine with military aid and, should there be a ceasefire, would then help Ukraine with longer-term defense.

On social media platform X, Ukrainian President Volodymyr Zelenskyy also wrote of his gratitude to Starmer.

Starmer was a very active and reliable partner within the "coalition of the willing," confirms Olivia O'Sullivan, director of the UK in the World program at British think tank, Chatham House.

O'Sullivan notes that Starmer worked with other leaders to consider how Europe could play a strategic role in securing peace even though he struggled to find the funds needed to meet his self-imposed military spending targets. But that's a challenge that the next British prime minister is also going to face, she points out.

Helping set EU policy
Britain also played a major role advocating for Ukraine within what is known as the E3 format. This framework includes Germany, France and the UK and is supposed to help coordinate the foreign and security policy of all three.

O'Sullivan says the E3 is behind a lot of European strategy, not only regarding Ukraine but also in dealing with the US government, which is increasingly playing a less significant and also less predictable role in European security.

For example, the E3 format has been used to make statements in response to US demands on how Europeans should contribute to securing the Strait of Hormuz.

The fact that the Europeans will now need to adapt to a new British prime minister isn't too much of a setback, O'Sullivan says, although Starmer's departure might see some loss of momentum for the E3.

Who's next?
Exactly how the UK's relations with the EU and European leaders develop in the future is going to depend on who succeeds Starmer. Currently there are strong indications that the former mayor of Manchester, Andy Burnham, will become the next British prime minister.

Burnham is considered broadly pro-European, O'Sullivan told DW. She expects him to essentially maintain Starmer's course regarding the EU and Ukraine although, she says, it's hard to tell accurately as he has tended to focus most on domestic affairs. That is why it's also not clear whether Burnham would prioritize foreign policy in the same way Starmer did.

O'Sullivan says that the Starmer government's greatest achievement in foreign policy was likely the establishment of an EU-UK security partnership, which was formalized last year at the first EU-UK summit since Brexit.

Starmer had campaigned for a "fresh start" with the EU and had already made gains towards that in some areas, like youth mobility and exchange programs as well as in the fisheries sector.

EU-UK summit postponed
The new "Security and Defense Partnership," which was signed off on last May, is intended to facilitate further cooperation on Ukraine and on other security and defense initiatives, as well as providing opportunities for regular talks.

O'Sullivan adds that this success was somewhat diminished by the fact that the UK only got limited access to the EU's SAFE (Security Action for Europe) program, a €150 billion financial instrument designed to speed up defense readiness in member states. The UK was unable to fully participate in that last November because the British were unwilling to pay a participation fee.

There is no doubt that the UK's relationship with the EU will be something that next British prime minister will be grappling with soon enough.

In fact, the EU seems to be waiting for a new British leader to do so. On Monday, Antonio Costa, the president of the European Council, announced that an EU-UK summit originally scheduled for July 22 had been postponed for the time being.