I would first check with your County Wildlife Trust to find if the woodland has any designations ie; LWS/SINC local wildlife site/site of importance for nature conservation…….or higher ().
If its not designated, then assess the age of the deciduous trees in the wood, if they are old, or there are old rotted stumps in the wood then it is probable that their roots have gone down a long way and this would make the task of creating a ‘pan’ in the bottom much more difficult, but not impossible.
Next go to your local council and ask to see their geology maps of the site, if they show underlying rock it could be difficult creating a waterproof seal in the ‘pan’, if its shows clay, especially ‘blue’ clay your on a winner
Remember that anything to do with water requires EA consultation and consent, and you will certainly require ‘change of use’ planning permission from your local council.
If you get beyond this point with an all clear, pm me and I take you through all the stages to filling.
Reckon on two years from start to finish as an absolute minimum.