Seem to remember someone posted a photo of a Lancia in a museum a while ago, a possible contender? My money would be on that or one of the Austins in Russia. Oldest British one is surely the Peerless
There's a Peerless at Bov & one in Ireland. Fiat-Izorski Armoured Car Central Armed Forces Museum, Moscow
A Russian Austin at the Military Historical Museum of Artillery, St.-Petersburg. A Lancia 1Z, Museo di guerra per la pace Diego de Henriquez, Trieste. That was modernised in 1930.
Second original Russian Austin is as memorial on the terrain of Izhorsky work in Kolpino (St. Petersburg) - Izhorsky work assembled more Austin, as Poutiloff Work.