Lance,
Pretty nigh impossible for an individual to import trees to the UK I would say.
The big UK importers go on a buying trip in April/May, select the trees and ship them in a couple of months later where - depending on the source country- they remain in 6 month quarantine on average.
Prior to being shipped they will have had to have been grown for 2 years in an approved nursery and have a phytosanitary certificate issued.
In the meantime they often sell them from photographs taken prior to shipment and you can collect in early springtime the year after generally. That's how Windybanks used to operate for instance.
Certain things can speed up import a tad such as soiless "soil" that can be sterilised more easily, pre-shipment fumigation and certification.
Full text of the Defra explanatory leaflet is
Here
TimR