The main problem with the sites linked to is that they are too small, and so very expensive per acre. They are within the reach of individuals to buy for "amenity" use, that's why they are for sale in this way. It's an easy way for landowners to raise a bit of cash from land which is otherwise useless to them. A deciduous woodland of a few hundred acres would be a lot cheaper per acre, say £1-2,000 per acre. Unfortunately, such areas of land are rarely sold unless as part of a larger estate which could include farmhouses, cottages, sporting income and farmland. Landowners are normally reluctant to split such estates, as they are very difficult to assemble in the first place!
A more realistic "group buy" approach would be to buy the rights to a wood (or a series of different woods scattered around the country), not the wood itself. A landowner will probably make less than £100 profit per acre per year from farming, and nothing at all on woodland. So if a 50 acre wood could be leased for £50 per acre per year, this would give the landowner £2,500 p.a. income. If 25 people all paid £10 per month by standing order, it would more than cover the rent, and 25 people would be sustainable in that size wood. If 100 people paid in, you would soon accumulate a big enough income stream to lease a second wood in a different location. As more people joined the scheme, a greater choice of different venues would become available, for the same £10 per month payment. You could conceivably end up with a network of venues around the country available to members, in return for their annual subscription. Anyone who wanted to leave the scheme could do so, but would no longer have the right to use the woods.
There are lots of opportunities for abuse by those not prepared to pay, but nothing's perfect! It could be set up so that those unwilling to commit to the subscription could pay as they go, say £10 per day. There would have to be a company to take the leases, of which the members could be shareholders, appointing a board to run things, and voting on new purchases at the AGM...
In one of my lives I'm a property consultant by the way!