Lady Thatcher's coffin is due to be taken from Parliament by hearse to the nearby church of St Clement Danes, where it will be transferred to a King's Troop Royal Horse Artillery gun carriage.
Processional minute guns will fire at the Tower of London as the carriage heads to St Paul's.
Argentina's ambassador to London Alicia Castro has declined an invitation to attend the funeral, in a mark of continuing tension over the Falkland Islands.
Downing Street has also insisted Mr Cameron is not disappointed with Washington's decision not to send currently serving senior politicians to the funeral.
Barack Obama's official delegation will be led by George Shultz and James Baker, who both served as secretaries of state during the Thatcher era.
Former US vice-president Dick Cheney and ex-secretary of state Henry Kissinger are also due to attend in a personal capacity.
French President Francois Hollande will be represented by Elisabeth Guigou, a close adviser to Francois Mitterrand while Lady Thatcher was in office.