The 54-year-old sergeant was shot in the chest multiple times by a handcuffed man in Croydon Custody Centre, South London, in the early hours of Saturday.
Ratana, who hails from Hawke's Bay and worked five years for Auckland Police, was approaching the shooter to perform a Covid-19 temperature test when he was shot at point blank range.
London's Metropolitan Police announced overnight that they had arrested a man on suspicion of supplying a firearm.
The man was arrested early on Sunday in Norwich and is being held at a South London police station.
The suspect in Ratana's shooting remains in hospital in a critical condition and police have not yet been able to interview him.
Meanwhile, friends and teammates at East Grinstead Rugby Club have gathered to remember Ratana, who was a senior coach at the club.
The club's flag flew alongside the New Zealand flag and an All Blacks flag as two periods of silence were held for a man described by assistant head coach Ryan Morlen as an "irreplaceable figure", the Press Association reported.
The New Zealand family of Ratana told The New Zealand Herald yesterday they were "devastated", and Prime Minister Jacinda Ardern posted her condolences on Facebook, saying she was "incredibly sad to hear the news".
The father-of-one was only two months off retirement, and previously helped guard Princess Diana, the Queen Mother and a former prime minister.
The BBC reported the suspect was known to UK counter-terrorism officials and aspects of his background may feature prominently in police inquiries.
The 23-year-old suspect is understood to have shot himself after opening fire on Ratana.
He had been arrested for an alleged drugs offence and possession of ammunition.
Despite London Metropolitan Police commissioner Dame Cressida Dick saying the shooting was not terror-related, the motive for the shooting is expected to be the focus of the police investigation.
A UK police watchdog said the suspect had his hands cuffed behind his back, and they are examining CCTV and police body cam footage to establish how the shooting happened.
Of particular concern was how thoroughly the suspect was searched before he was taken into custody.
The family of Ratana's partner of four years, Sue Bushby, was reached by British press over the weekend.
Bushby's sister, Amanda Tessier, questioned the procedures in the South London police station that let this happen.
"We simply can't believe it. How did someone have a gun in the police station?" Tessier told The Sun.
"I'm sure there is going to be a huge investigation by the Met but it doesn't seem right at all."
Tessier said her sister was distraught at the news.
"She got a knock on the door in the morning. It's just devastating," Tessier said.
"We can't believe it. He was the life and soul, a real fun-loving guy. He was a great big friendly bear of a man, one of the loveliest men you could meet."
Originally from Hawke's Bay, Ratana was educated at Palmerston North Boys' High School and went to the UK in 1989 while in his early 20s. He joined the London Metropolitan Police two years later.
He was once just 300m from an IRA bomb that exploded outside the British Prime Minister's residence at 10 Downing St in 1992.
Ratana returned to New Zealand to serve for Auckland City and Counties Manukau police between 2003 and 2008.
NZ Police rang Ratana's sister in New Zealand yesterday morning to break the news.
"Everyone is really devastated to hear the news he has passed," Ratana's cousin, Adrian Rurawhe - who is the Labour MP for the Te Tai Hauāuru electorate - told the Herald.
"He was really proud to be a police officer, he was also really proud to be Māori from New Zealand."
Rurawhe described Ratana as "fearless".
"We all go through a journey of discovery, especially those of us that didn't grow up with te reo, but he was so proud of being Māori."
Ratana also had strong ties to New Zealand history. He is a grandson of Iriaka Ratana, the first Māori woman MP, and great-great-grandson of Ratana Church founder Tahupotiki Wiremu Ratana.
Ratana was also heavily involved in rugby throughout his life, becoming senior coach of East Grinstead RFC, south of London, in recent years.
The fallout from Ratana's death was reverberating in the UK police community last night, with a former head of Manchester Police telling the BBC the shooting was "obviously going to cause a lot of fear for police officers".
"It's very upsetting for them as it makes them feel so vulnerable," former Chief Constable of Greater Manchester Police, Sir Peter Fahy, said.
Fahy added the circumstances around Ratana's death were "incredibly unusual".
"There are procedures of searching prisoners before they are taken to police station in vans or in cars, they're searched again when they get to police stations and there are additional procedures because of screening for Covid."
UK police officers were also yesterday turning their social media profile pictures black with a blue stripe to pay their respects.
Born to a Scottish mother and Māori father, Ratana was the eldest child of an extended whānau of stepbrothers and stepsisters.
His immediate New Zealand family told the Herald they did not want to comment.
