The hose pipe ban doesn't prevent you from washing your car, just from using a hose pipe connected to the mains water. Although a bit of a bother but if you are that fussed, give it a good clean then maintain it daily from buckets (provided it's not really dirty, just dusty) or you'll just have to grin and bare it.
For bird poo I dampen some kitchen roll and place it on and leave it to soften the poo and lift off with more wet roll. Not ideal but needs must.
Cleaning in these hot conditions isn't pleasent anyway so I tend to leave it a while between washes.