By the time the water issue become very serious in England, there would be other far more serious issues, globally and locally, that will dwarf the water issue. For instance, water issue around the world would drive the people out, water wars breaking out, drought and food crisis, powerful nations simply taking over resource-rich lands globally and so on. A swing back to medieval times.

On the other hand, dried-up England will not be a target for migrating to so that's a net positive for the current political constellation.