The couple – who undertook a summer road trip across the U.S. together – gave each other a kiss and then walked around British Style Observed, an exhibition and auction of photographic works by McCartney's daughter Mary, 39.
Shevell, 48, told PEOPLE that her favorite was a photo of The Queen Best Actress Oscar winner Helen Mirren.
Held at the Natural History Museum, the event benefited Macmillan Cancer Support, which assists those with the disease.
McCartney, 66, is gearing up for his "Friendship First" concert in Tel Aviv next week. And despite reported threats against his life by militant anti-Israel factions, the former Beatle says he's keeping to his usual show preparations.
"It's the same as always, I'm rehearsing," he told PEOPLE. "I was a Boy Scout. 'Be prepared,' that's the motto." In a public statement about the threats, McCartney said: "My message is a peaceful one, and I hope that the idea will spread."
Stella Attends with Kate HudsonAlso at the British Style event, part of the National Magazine Company's 30 Days of Fashion and Beauty Festival, were McCartney's younger daughter, designer Stella McCartney, 37, and Kate Hudson, who arrived and left together.
Six photos by Mary, 39, were sold to benefit the cause. Afterwards, Mary – who gave birth only five weeks ago – showed off baby photos on her iPhone and told PEOPLE that she named her newborn son Sam.
As for the sale of her photos, "I think [the ones of] Stella sold for the most!"