Actually, according to flag signals, these vehicles are approaching a camp. The left shown flags means "form a camp".
But I might be wrong on that.
Well, you said its British hence this would exclude any ANZAC formation.
I was looking at the colonial helmets and the head lights of the vehicles.
Actually the helmets would exclude an early WWII event, and the head light would exclude a WWI action, I saw pics of these armoured cars at Gallipoli, in Fiandres and even in Dublin but they hadn't those head lights.
Hence I guess we are talking about late 20's or 30's in the Middle East (Syria, Palestine or even Iraq).
And since the Rolls Royce were "officially" phased out by the Royal Army in 1922 I guess those might be RAF personnel.
This is my guess, but I might be totally wrong!
Come on Omega, tell us!