National Collection Holders have a statutory exemption from water use restrictions, such as hosepipe bans, for watering the plants in their collections. However, in using water, it is requested that collection holders use water wisely and adopt water efficient practices.
The exemption for National Plant Collections is explained in the Water Use Order 2010
The water use restriction does not include watering plants on domestic or other non-commercial premises using a hosepipe if:
(i) grown or kept for sale or commercial use, or
(ii) that are part of a National Plant Collection or temporary garden or flower display.
“plants” includes plant organs, seeds, crops and trees.
Full text here (page 3)
To find out more about temporary bans on water use please refer to the Flood and Water Management Act 2010 here (page 22, section 36). The most recent guidance to water companies on water use restrictions 2013 is here - Managing through drought (page 20 no.3)