You need to be honest with yourself and forward thinking about what you want from the woodland. Woodland conservation for wildlife is satisfying, but it might not be so compatible with making an income from the woodland to subsidise your earnings or pension. There are also tax implications which differentiate between amenity and commercial woodland and grants which do the same.
If you are fit and have plenty of time you can make money from a mature commercial woodland, the standing timber could be worth close to what you pay for the land if you can supply the substantial amount of labour to cut and extract it. Amenity woodland is rather different and you are unlikely to get your money back for many years.
I see you are in London, so you'll probably be looking for land in Kent, Surrey or Sussex. Most commercial woodland in these areas is Sweet Chestnut coppice, a very dense woodland not ideal for wildlife due to the close canopy, mono-culture and it being a non-native species ( although it's been here since the Romans ). On the plus side; they are often planted with Oak "standards" ( full size trees not cut on coppice rotation ) which are host to 250 species of plant and insect, and are considered commercial woodland eligible for all manner of grants and tax-breaks.
If you do buy a commercial woodland don't forget that the age of the standing wood should be reflected in the price of the land; SC coppice will need to grow for at least 20 years before you get a good crop so if it's been cut the year before you have a long wait, and you know why it's up for sale ( you will, however, have the most amazing wildflowers springing up between the coppice stools). If you can find one 20+ years since the last cut and at a reasonable price you are onto a winner, depending on soil and planting density you could have 80 tons of post and rail fencing and 50 tons of firewood per acre.
I was talking to a forester last week who was complaining that many large woods are being broken into smaller plots and sold, a lot of members of this forum have bought these, and the forester was saying that it is becoming difficult to cut these woods now because the owners don't always see the trees as a crop.
I can see his point to an extent, if a wood was 100 acres and was normally cut at say 5 acres per year the coppice gets to grow for 20 years each time and the specialised wildlife that lives in coppice of a certain age has an easy time following the suitable habitat. Now imagine the woods has been divided into 20 lots, which owner will want to have his 5 acre lot cut completely ? Will the neighbours allow the logging trucks to cross their land to extract the timber ?
Of course some woods are so neglected that the smaller owners perform much needed maintenance, but you can see that what I'm saying is that you need to have a flexible attitude and be aware that sometimes cutting a tree down is the best thing for it.
Lastly, as Tracy says, the closer the better.