The couple joined veteran actress Barbara Windsor and members of military charity the Royal British Legion for the trip across the city in a 1960s-style Routemaster bus ahead of Remembrance Day this weekend (10Nov13), which commemorates soldiers who have died in battle.
The royals were also photographed at London's High Street Kensington Underground Station, speaking with military personnel and buskers who were fundraising for the cause.
Poppy Appeal volunteer Tim Connolly, who spoke to the couple while collecting funds at the station, tells Britain's Daily Mirror newspaper, "It was a surprise. We were told we would be having Vips here today but not who. It was a shock, very much so. William picked up the tin and saw it was full and said 'Well done'."