Tthe 29-year-old man and the 31-year-old woman were arrested in connection with investigations into Tuesday's armed raid on the
Securitas main cash depot in Tonbridge, southeast England.
A 100-strong police team is engaged in the hunt for the gang of at least six armed robbers and an unprecedented two million pound (A$4.74 million) reward was being offered for information as to their whereabouts or identities.
All British ports and airports have been put on alert for anyone attempting to leave the country with large sums of cash while CCTV images of the gang's white delivery lorry was released to help track those responsible down.
The raid, probably the result of months of extensive reconnaissance work, saw the depot manager and his wife and eight-year-old son abducted separately and 15 workers held at gunpoint.
They were later identified by Britain's domestic Press Association news agency as Colin Dixon, 51, his wife Lynn, 45, and their son, Craig.
The kidnappers, who forced Mr Dixon to help them gain access to the cash, were disguised as police officers while the others, possibly up to six, were said to be masked and wearing boiler suits.
Kent police said it was "an obvious line of inquiry" that the raid was conceived and carried out with inside knowledge and he was keeping an open mind into what happened. The money was a mixture of used and new banknotes.
Biggs haul
He also could not say whether the gang may have fled via the Channel Tunnel 80 kilometres away. Thursday's Daily Mail newspaper said CCTV footage was being checked to determine if they escaped to France.
Kent police are also in touch with police in Belfast, Northern Ireland, where 26.5 million pounds (A$62.8 million) was stolen in a raid at the headquarters of the Northern Banks, making it the biggest cash theft in British or Irish history.
In comparison, Ronald Biggs and his accomplices netted about 2.6 million pounds (A$6.2 million) in the infamous "Great Train Robbery" of a London to Glasgow mail train on August 8, 1963.
Foreign police, Scotland Yard, plus Britain's National Crime Squad and National Criminal Intelligence Service, which deal with serious and organized crime, have also been informed.
Depot staff raised the alarm about an hour after the gang left. None was injured and were being interviewed, police said.
Detective Chief Superintendent Paul Gladstone, from Kent Police, said earlier Thursday that the robbers were "armed, dangerous and violently threatening" to kill Dixon and his family.
The Bank of England said its governor had asked for a review of security arrangements at depots such as Securitas, which provides security guards, alarm systems and cash transportation services.