Leigh,
Look at woodlands.co.uk - this is the major outlet for woodland sales in the UK. I don't know if there are any indices such as you get for agricultural, or commercial property from the likes of the Royal Institution of Chartered Surveyors (but if you can navigate their horrific website search system you might find something - possibly called "woodland market survey"..?)
As a Land Agent I keep my own records of property sales going back several years - my experience is that woodland prices have been gently rising for the last 4 or 5 years - mainly due to the dreaded "lifestyle" purchaser...!
If you are thinking of investing for "owning property" woodland is a good bet, there's grants to support ownership/management, there almost certainly always will be, there's timber sales which should only ever get better (what with peak oil, etc) and ultimately land (whatever's growing on it) it is a finite resource...
Jamie