Desalination plants are expensive to run in terms of power. I did read it would around 1MW to desalinate enough water for the average 4 person UK house (approx £100) plus the cost of building the plant, maintaining it, etc
Plans to build a new reservoir in oxfordshire turned due to pressure from local residents
Plans to build water network from north to south in the 70's were rejected
of course there is a plenty being wasted through leaks as well
BBC News - Drought summit: Why not pipe the water from north to south?