"I told him I was going to propose and he said 'Many congratulations in advance'," Jackson said.
Jackson, 28, of Newbury, Berkshire, emerged from the palace with the cross while Rebecca Daniel, 29, of Budleigh Salterton, Devon, wore a solitaire diamond ring.
"It was totally unexpected," she said. "It's a day that's not going to be trumped for a while."
Jackson was honored for throwing back a grenade while serving in Afghanistan last year. He also used his own body to shield a comrade from the explosion.
He described himself as a "baby Marine" when he went to Afghanistan.
"There are so many stories out there that are untold -- so many lads that deserve this," Jackson said.