The tanks provide you with the head of water (pressure at your taps)
the higher the tanks the better the pressure.
They really need to remain in the loft but moved into the eves
Normally with a loft conversion you have vertical walls behind which there is dead space.
You may find that you have to fit long and low (coffin) tanks to get the same of stored water.
Other options...change your immersion cylinder for a mains pressure type (no header tank required)
You also have the option to change you boiler for a combi type (no header tanks and the immersion cylinder goes as well