That fount of knowledge(?) Wikipedia, has a fair amount of info on the tree.... http://en.wikipedia.org/wiki/Glastonbury_Thorn
Interesting that it only grows true to form from cuttings or grafts and not from seed. Any trees are therefore exactly the same, they are clones. If HM the Q strikes a few hardwood cuttings from her Balmoral Christmas display she could have a whole forest of the things in a few years time.
Jim.