Although the colour is similar to the green paint used internally on many WWII British aircraft, the filler cap is the same as that found on a Boeing Stearman, so this cap is not a British design. Also you aren't going to get much flow out of this tank. If you assume that the filler neck is upright when filling of the tank is required then the small fitting adjacent to the filler neck can be considered to be a vent fitting or alternatively a pressure fitting that would allow a higher or lower internal pressure to be introduced to the tank than the surrounding atmospheric pressure. The spherical ends on this tank tend to lend themselves to this being a pressure or vacuum vessel. Also, if the filler neck is upright then the fitting on the opposite sidewall can be considered the outlet (or inlet). Looking at the size of that fitting you wouldn't get any sort of decent flow out of that small opening (guessing the diameter of the hole in this fitting at being no bigger than 1/8 inch). The seal on the filler cap would only work efficiently at low pressure differentials. My take on this vessel is that it is some form of a low pressure tank or accumulator. It's thick laminated side walls and curved end caps make it something different to just being a water tank.
Thanks very much for the detailed input. The owner claims that it is an emergency potable water tank which might explain the small outlet. The dished ends, in my opinion, may be due to buckling over time because of shrinkage of the cylindrical part and so may not be original.
What I struggle with is the "heavy" nature of this construction. I doesn't seem to lend itself to a light aircraft like a Stearman.