There's organizations working towards reforestation in Scotland. An interesting take is Norway has similar problems with farmland taking away old growth, while also being at such a northern latitude.

But in the last century much progress was made there (norway) in reforestation, so there is a frame work for Scotland to do the same.

The deer problem there is also exasperated by their Hunting regulations. It boils down to not having public hunting grounds (hunting isn't included in their right to roam law as far as I'm aware). So private estates that hold huge swaths of land basically hord all the Stags and charge $$$$$$ to anyone who wants to shoot them. So unless your extremely wealthy, you ain't culling the herd.