since it is not likely that the garden of Highgrove will be open for the public any time soon, second best vist the garden of Hanham Court, open for the first time this year. It belongs to gardendesigners Julian and Isabel Bannerman whose work can be found at Highgrove.
Hanham Court Gardens - Rural Manor house between Bath and Bristol
here is a bit of Highgrove:
http://www.bannermandesign.com/