Thats the British Standard, haven't read it, so can't comment but would be very surprised if it was not based on the same roots as NATSPEC ie the Florida and Californian models.
Go here
http://www.doacs.state.fl.us/pi/pubs.html scroll down the list on the left to Grades and standards..tree section divided into 4 files and below the palms and cycads section, in these pdfs you'll see exactly what is being defined as best practice in the nursery and how to recognise it in the plants you might consider buying.
As I've said previously this is for Florida but the principles established there have been adapted into NATSPEC and are shared no doubt by the British standard.