From the OS OpenData’s Meridian 2 dataset, which was released under a free licence today, here is the extent of forest cover across Great Britain – there is a dedicated polygon shapefile within the distribution showing just this:

As a general rule for orienteering, areas with good forest cover have the best orienteering maps. Scotland and Wales beat England hands down for cover, although Surrey’s doing not too badly at all. The rule doesn’t always hold though – there’s a big patch near Cambridge – more so, it appears than the Lake District, but the latter is considerably finer for orienteering in.

The map contains Ordnance Survey data © Crown copyright and database right 2010.