Not sure of the current UK standard, I will chase it up but think Ekka will beat me to it. Over here there are three valuation methods accepted in court, depends which state you're in and the LGA. We have a national standard that has not come out of the draft form as yet.
Revised Burnley trunk method is accepted by courts here in Queensland, all the methods follow very similar processes put very simply numerical value is assigned to various measurements of the tree relating to its health and vigor, its SULE, its cultural/botanical importance etc..and usually have a base unit value calculated either from the CPI or a $ per cubic metre of tree at the heart of the calculation.
Our standards for valuation have followed those in the UK so I can guarentee you can value any tree you want in the UK and have that valuation stand up (provided its done by a qualified person) in court.
SF
If you can afford the time and money the Consulting Arborist Society is running a seminar 27-28 April 07 on the very topic here's the details of the seminar and the AGM;
AGM-capel.pdf
SF
