"I got very drunk at one point, and I was determined to ask him to be a godfather to my daughter," the actress, 47, recalled this week at a glamorous Windsor Castle charity dinner to benefit the Royal Marsden cancer hospital, of which William, 31, is president. "He quite wisely said 'no.' "
Bonham Carter said she cheekily popped the question when she met William and his then-girlfriend Kate Middleton while they were vacationing on the Caribbean island of Mustique.
Bonham Carter added, "I admire him. He has got the same grace as both his parents."
At the Windsor Castle gala, fashion titan Ralph Lauren announced he is funding a new breast cancer research facility at the hospital. Besides Bonham Carter, celebrity attendees included Emma Watson, Cate Blanchett and Kate Moss, who wore Ralph Lauren.
The 200 guests enjoyed an elegant dinner of seared hot-smoked salmon, green beans and sugar snaps, roasted fillet of beef with caramelized shallot and tarragon butter, and steamed English asparagus, followed by a chocolate and salted caramel tart.
Sherlock actor Benedict Cumberbatch said of meeting the Prince: "He is just a stand-up guy. I completely forgot to ask him how he got over his jet lag from his trip to Australia, because I was there at the same time and I saw all the commotion when he went to Sydney Opera House."
At the dinner in the castle's Great Hall, William sat between Ralph Lauren's wife, Ricky, and Royal Marsden chairman Ian Molson, while the designer sat across from the Prince and next to Blanchett.
Also seated near William were David and Julia Koch, the American billionaire philanthropists.
Earlier, William told Lauren, "It's a wonderful thing you're backing. It will be another great addition to the hospital."