A ceremony was held in Yorkshire after an inquest in Surrey last week found Oliver Gobat was unlawfully killed.
His mother, Helen Gobat, said: "I don't think there'll be any closure until there's some sort of justice."
The body of Oliver Gobat, 38, was found in a burnt-out car on the Caribbean island in April last year.
He was reportedly shot and then set alight.
Mrs Gobat, from Esher, said the past year had been "a living nightmare".
She said the scattering her son's ashes was "moving" and brought distress rather than closure.
"We've just got to learn to live with his loss, but certainly the lack of progress on any justice for Ollie is absolutely crippling and agonising," she said.
She said it took six months to get her son's body back to the UK and then several weeks before he could be cremated.
The family had been calling for British police to take a fresh look at the investigation, she added.
After last Thursday's inquest, the Foreign Office said it had been agreed in principle UK police could assist investigators in St Lucia.
Mrs Gobat said the family had theories about what had happened but could not talk about them at this stage.
Mr Gobat, known as Ollie, was born on the island but grew up in Surrey, where he played junior cricket at county level.
He and his two brothers ran a luxury hotel at a resort in St Lucia.
The family of a hotel owner who was murdered in St Lucia have scattered his ashes, saying they have to "learn to live with his loss".