Does anybody know of a particular model of "undersink" type low-power
electric water heater which is designed for a low-pressure water system
with header tank, not mains pressure?

I seem to be struggling to see anything online which clearly states that
it is (a) for water systems at header-tank pressure only and (b) does
*not* have hot water storage but only provide instantaneously heated water.

Any particular model names much appreciated.

Michael