The midday service culminated the day's big moments in London, beginning with a Dawn Service at the Australian and New Zealand war memorials at Hyde Park Corner, followed by a wreath-laying ceremony at the Whitehall Cenotaph.

Kate was surprisingly joined by brother-in-law Prince Harry, who was not listed on the official program.

Commentators mused that Harry must have made a last--minute decision to attend after spending time caring for his pregnant wife, Meghan, who is due to give birth imminently.

The Australian, NZ and Turkish flags stood side by side with the Union Jack on the church's altar throughout the packed service.

The Dean of Westminster Dr John Hall prayed for an end to terrorism and for peace

"The spirit of national pride encourages us, as we bring to mind in particular the recent terrorist attack in Christchurch, New Zealand," he said.

"We pray for an end to terror and for the triumph of peace."

Presumably in a reference to Brexit, Dr Hall also expressed his "personal opinion" that the bonds between Britain and Commonwealth countries like Australia, NZ, Canada and South Africa should be strengthened.

About 2000 people turned out at the Dawn Service, where Australian High Commissioner George Brandis told them 2019 marked the centenary of most servicemen coming home from World War One.

He said this year's commemorations would focus on the mental scars of war like post traumatic stress disorder, known after WWI as "shell shock".

"For both of our nations, Australia and New Zealand, the 1920s were a blighted decade, though so much of suffering went unobserved, borne with grim fortitude behind the doors of quiet suburban homes and peaceful farms," Mr Brandis said.

"The physical injuries were manifest and in most cases they would heal. What was not visible or even understood was the mental suffering of so many of those who returned."

Brisbane local Melanie Roylance, 54, attended the service to remember many family members, including her father who died of complications years after his World War Two service.

She was impressed with the government's decision to highlight the suffering of returned service people.

Ms Roylance said focusing on not just the individuals who died, but also the broken ones who returned and their families puts a human face on Anzac Day

"We've got relatives who came back with severe gunshot wounds and who were disfigured. One fellow was hit by machine gun fire across his face so he was badly disfigured, so he was shunned by people," she told AAP.

"People would see him in the street and look away. Even though they knew what had happened to him and why, he was still shunned by the society that he had gone off to support.

"So I think it's important that people think about those aspects these days rather than just people charging the hill at Gallipoli; there's a lot more to the war than people think."

Other Anzac Day services were held across the UK, in Bath, Cambridge, Manchester, Norwich, Staffordshire, the Channel Island of Jersey, Edinburgh and Aberdeen.

Australian Associated Press