Hmm. I was wondering if the National Trust would object to plantings -- I dunno if you can do that in national forests here in the US -- but it looks like they do plant stuff:

https://www.nationaltrust.org.uk/support-us/plant-a-tree

Help plant more trees

For only £5, you can plant a tree that will tackle climate change and support life for years to come. Your support will help to plant and establish 20 million trees by 2030.

https://www.theguardian.com/environment/2022/nov/03/national-trust-to-plant-1200-hectares-of-flower-filled-grassland-in-devon

National Trust to plant 1,200 hectares of flower-filled grassland in Devon