There is no reason why you should not fit a copper cylinder complete with immersion heater in your loft.
Providing the water in the cold water tank is higher than than the outlet of the copper cylinder it will work.
If there is little difference in the height then using 28 mm pipe to get the maximum volume of water will improve things.
If you have a shower, then you may need a new low pressure mixer.
You can buy copper tanks that lay on their side on a cradle, this may help solve any height problem.
Placing the tanks over a solid wall may help you to feel happier abut the extra weight in the loft.