First, there is no national policy on exemptions. It's up to each individual water authority to decide their approach. In recent years, most water authorities have permitted disabled people to carry on using hosepipes, but if your provider introduces a ban you should always check first to make sure you're exempt. Water authorities also reserve the right to impose the hosepipe ban on disabled people too if the drought becomes more severe.
Second, the exemption doesn't include all disabled people - just those who, in the words of some of the water companies involved, have "severe mobility problems" and hold a Blue Badge. The Blue Badge requirement doesn't, however, mean you can use a hosepipe to wash your car - it's still only to be used on gardens, plants and the like. It's also strictly limited to your own property, so if you were thinking of hiring yourself and your hosepipe out to the neighbours, think again, though in this case that doesn't seem to be a problem