The ceremony on Sunday marked the 85th anniversary of the victory by Royal Air Force pilots and aircrew during the Battle of Britain in 1940.
It was the first decisive battle in history fought entirely in the air, and was a dramatic turning point in the Second World War.
The Prime Minister attended the service with his wife, Lady Starmer, and they were greeted by members of the Royal Air Force. Leader of the Conservative Party, Kemi Badenoch, also attended.
After the annual service there was a Second World War-era flypast over Westminster, featuring an Avro Lancaster heavy bomber.
Ahead of the service, Sir Keir said: “Today, we pause to remember the heroes of the Battle of Britain.
“Those who stood firm in the face of unimaginable odds to defend our skies, our freedoms, and our future.
“Their courage was not just a moment of military brilliance, but a profound act of national unity and resolve.
“This date marks more than a turning point in the Second World War, it reminds us of the values that bind us together: duty, sacrifice, and the belief that Britain is at its best when we stand shoulder to shoulder.
“Their legacy lives on in the peace we enjoy today, and in our shared responsibility to uphold the freedoms they fought for.
“We will remember them.”
The Royal British Legion’s national chaplain said it was “worth remembering” that some Battle of Britain pilots were “immigrants who fled to Britain to join the fight for freedom, decency and tolerance”.
Giving an address at the service, the Venerable Dr Giles Legood said: “We live in a rapidly changing strategic environment where the Royal Air Force is often the first responder on behalf of the nation.
“We can look to heroic stories of those who’ve worn the RAF uniform, and we can learn from them.
“In the Battle of Britain, for instance, pilots from 15 different nations flew with the Royal Air Force.
“And in today’s febrile public discourse, it’s worth remembering that many of them were immigrants who fled to Britain to join the fight for freedom, decency and tolerance.
“Their difference was a strength and not a weakness.
“In their unity of effort, it was diversity and not uniformity which bound them together and brought them to success.”
