Mike your idea wouldn't work very well with a combi boiler. You'd also find with a loft tank system that unless the boiler is active the water will just get colder as you work
Also valid point about stupid taps, would be a hell of a fiddle getting water from indoors.
Plus the possibility of flooding (properly flooding) a customer's home is too great.
But maybe your idea has legs, with a few modifications? You could connect from a cold feed (outside tap obviously is ideal), feeding it back to the machine to be pumped - after all it would be more handy to have all the controls in one place and the unit in one piece.
Such a system would probably would need a header tank to avoid starvation....maybe send the water through a heat exchanger...
oh hang somebody already thought of that! They called it a "truckmount"