I guess it wouldn't be practical to put the water softener in the loft
due to the frequent need to refill with salt?

The problem is putting it near the water path to the outlets that you
require to be softened. In most houses, the incoming water main passes
through the kitchen and outlets are taken off either directly or via a tank
in the loft.

If you have a loft tank, the water softener can be anywhere on this line,
including the loft. If the cold water outlets are taken off at mains
pressure before the loft, the softener must be installed before those take
offs (but after the kitchen drinking tap and the outside tap).

A modern metered softener doesn't actually use much salt. I wouldn't
hesitate to put one in an easily accessible loft, except that my house is
entirely mains pressure, so the softener must be placed in the kitchen. When
repiping the kitchen, I used a three pipe solution, with hard cold, soft
cold and soft hot pipework. Unfortunately, I can't actually afford a
suitable high flow softener ATM, so the soft pipework is hard.

Christian.