This question refers to gardeners of a commercial nature and includes landscape gardeners. We suspect it may be relevant to other industries too. As with all questions related to the hosepipe ban, answers can be open to interpretation and rules are enforced differently by each water company. If in doubt, contact your local water company.
The general rule about this at the time of writing (2012) is that businesses are exempt from many hosepipe ban rules. However, in 2012 no business is allowed to go to a private property and water gardens with a hosepipe.
All seven water companies involved in the 2012 hosepipe bans have now altered their rules for professional gardeners. Please see this post for more information: http://www.hosepipeban.org.uk/2012/05/23/gardening-businesses-now-exempt-but-bans-continue/