Point them in the direction of the National Archives (of the UK), of which even a cursory look which will reveal the Board of Ordnance didn't start issuing camp equipment (including canteens) until 1821. Between 1808 and 1821 camp equipment was issued by the Storekeeper General's Department, and before that the Commissariat.
But I guess some people are stuck in their ways: Osprey books still claim the British wore knapsacks with wooden frames in the Napoleonic era.