A royal flush is guaranteed at Westminster Abbey’s annual Commonwealth Day service on Monday. The Queen, as head of the 54-nation alliance, will be seated in the front row alongside the Prince of Wales, Duchess of Cornwall, and the Duke and Duchess of Cambridge.

But behind, a little apart from this nucleus of the royal family, will be the Duke and Duchess of Sussex, in their final official UK public engagement as senior working royals before “Megxit”.

The prime minister, senior politicians, and ambassadors from around the globe will be in the congregation to bear witness to Harry and Meghan’s last hurrah before they depart these shores for an uncertain future.

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The service draws to a close the couple’s brief farewell tour, which began when Meghan flew in from Canada, where they are currently based, to join Harry at the Endeavour Fund awards for wounded and sick military veterans at London’s Mansion House on Thursday.

If, during their flurry of engagements, there has been time for moments of reflection, or even a little sadness, at what they are leaving behind, there was no trepidation evident in their dazzling smiles to the camera.

According to a source close to them, the couple are on “great form”. Both are feeling “positive and excited” at what the future holds, even though events have been somewhat overwhelming since they made their shock announcement in January to step back as working royals and base themselves mainly in north America.

Facebook Twitter Pinterest ‘The Queen has left the door wide open [to Harry]’, according to the royal historian and author Hugo Vickers. Photograph: Samir Hussein/WireImage

They remain “1,000% committed” to their patronages, it is said. In the past couple of days they have had private meetings with organisations they are involved with, and with their staff. After 31 March, this team of 15 – currently based at Buckingham Palace – will either be made redundant or redeployed.

Meghan, who is said to be “very happy to be back”, has attended several private meetings. On Thursday, she paid a low-key visit to a hologram exhibit at the National Theatre, as well as a comprehensive school in Dagenham, east London on Friday. On Sunday she will mark International Women’s Day.

Meanwhile, Harry joined the Formula One champion Lewis Hamilton to open a museum at the Silverstone circuit, and met staff at his Invictus charity, to which he remains committed.

On Saturday the couple will attend the Mountbatten Festival of Music at the Royal Albert Hall. It will be Harry’s final engagement as Captain General of the Royal Marines, the highest rank of his honorary military titles, which he must now relinquish under terms agreed with the Queen.

Agreeing to sever his official military ties will have been painful for Harry, who spent 10 years in the army, and who on Thursday, told veterans he was “deeply proud to have served alongside you as Capt Wales”.

No replacements will appointed to these roles in the 12 months before the couple’s new status is reviewed. “Nothing is final,” said the source. “It was intentionally not binary … Because this is the great unknown. And, the family wants the best for them.”

This was the message the Queen conveyed to her grandson over a private lunch at Windsor Castle last Sunday. The lunch, at Harry’s request, was their first meeting since the Sandringham summit in January, where senior royals thrashed out details of the couple’s new financially independent role. But they have spoken regularly on the phone.

Facebook Twitter Pinterest Prince Harry meets Lewis Hamilton at Silverstone on Friday. Photograph: WPA Pool/Getty Images

“The Queen has left the door wide open. It’s almost as if she has seen that there is a good chance that their schemes won’t work,” said the royal historian and author Hugo Vickers.

The couple are expected to return to Canada, where they left their 10-month-old baby son Archie, shortly after Monday’s service. They will soon announce details of new non-profit organisations.

Though they will no longer be official working royals, this is not the last time they will be seen in the UK. They are said to be “absolutely committed” to serving their patronages, which would involve travel to the UK and the Commonwealth, though immediate plans may be affected by restrictions due to coronavirus, it is said.

Pictures of Harry, sixth in line to the throne, and Meghan seated behind William and Kate will dominate British media on Monday. “By virtue of being in the second row, it points out [Harry] is no longer third in line, as he was for quite some time,” said Joe Little, managing editor of Majesty Magazine.

“He will go further down the pecking order, as [William and Kate’s children] George, Charlotte and Louis grow up and have children of their own. And, rather than let time and Mother Nature play their parts, they have speeded up the process, by stepping back, as they are perfectly entitled to do so.

“As far as the Prince of Wales is concerned, he must be pretty devastated by all this,” Little added. Aside from his feelings as a father, Harry and Meghan were a key part of Charles’s plans for a slimmed-down monarchy. “I do think he must be very upset.”