Not sure about WW1, but in WW2 the Petrol can should be plain, but with 'Petroleum Spirit Highly Inflammable' with WD and a date on the top. No name on the side, and it would be coloured Kharki or OD. Water cans were identical, but the top was plain with no markings. All cans were dated on the bottom. Water cans were either painted White with a Black line and W or Water, or Black with a White line and W or Water (I've seen both in old photos). Brass cap was plain in all cases. Canadian manufactured cans were different, as they had a ridge around the top.
In WW1 I believe Shell supplied the Army with thousands (millions?) of cans, and these had Shell stamped on the sides. There is a reference to Shell Road which was made out of flattened Shell cans. I have heard that Water was sometimes carried in old Petrol cans in WW1 as there were complaints from the troops about the taste of their drinking water.
If the cans have a price stamped on them, this is the deposit the user paid to get the can. It also makes it Civilian and not Military.
£5 each isn't bad - about average price.
In WW2, Jerrycans used for water in the British Army had a white cross + painted on them so they wouldn't get confused with Petrol jerrycans.
Hope this helps.