I'm not sure I fully understand the configuration. If the unheated water storage tank tank is in the attic, is it inside the homes insulation, or outside? (In other words, is the attic insulation at the roof deck or is it at the attic floor?)
What is the temperature of the ground?
To pre-heat the water the pipe needs to pass through something that is warmer than the stored-water temperature. In the summer an uninsulated attic will be at temperatures well above that of the ground below, and routing the water underground will cool it off ahead of your boiler, but in the winter it would be the other way around- the attic would be cooler than the ground, and the ground would add some heat.
Hot water need to be at least 40-45C, and unless you live in an equatorial desert your ground temps are far below that. To raise the temp to useful levels requires either burning fuel in your boiler, or using a heat pump (air-source or ground source) to move heat into the potable water from whatever the heat source is (air or water), or using a solar panel, etc.