There is currently a consultation underway over future water resources for the Thames Valley. The main supply companies agree that we will need a new major reservoir, and have proposed a site near Abingdon. This would be a common facility for all local water companies. It could store more water for us and put it into the river system when we need it.
Demand continues to expand with a growing population. There are one off savings in volumes when people switch to water meters. The danger in not providing additional storage capacity would come if we had three low rainfall years in succession. There could also be problems in relying too much on withdrawing water from chalk natural reservoirs, and seeking to rely on too much abstraction from rivers. Both of these can have unpleasant environmental impacts.
The extreme case of Cape Town is a warning to those who think we should keep capacity close to demand. Cape Town is now suffering badly from water shortage thanks to failing to put in more storage, and has now experienced a long period of low rainfall.
Thames Water is running its desalination plant most of the time on partial capacity, though this was designed to be an expensive long stop provision for shortage periods.