National Council for the Conservation of Plants and Gardens or NCCPG is a charity that manages the system of National Plant Collections. Amongst other activities its member receive  a biannual magazine Plant Heritage and information on conservation.

http://www.nccpg.com/page.aspx?Page=1

The web site is informative and can help you locate where and when you can visit a particular national collection and lists open days and events. You can also purchase the Plant Collections Directory for £5.