I don't know anything about replacing trees, what makes this situation more complicated?
The location primarily, meaning high level of difficulty for access and within feet, literally, of Hadrian's Wall.
also the local rock is Whinstone - which is very hard / erosion resistant, and the soil is very thin so the rock is very close to the surface.
So, to replace a tree with something old enough to have grown to more than a whippy twig you have to dig a hole big enough for the rootball.
For a semi-mature tree, that equates to the diameter of the canopy.
You then have to dig up the new tree [there's a massive machine that does that] and transport it to the new location - whilst keeping the root ball from drying out. Did I mention that there isn't much in the way of a road access / around the site ?
What route there is, is also within the "protected area"
Next, you have to physically plant it, provide support against wind & frost rock, not to mention the local animal life that might try and eat it and finally supply water for some time [in case there's a lack of rain].
E2A - when they did some work on the paths for the Hadrian's Wall Trail in this area - the one ton dumpy bags were delivered using a helicopter !