I wonder if the subsidies necessarily make sense as structured.
Increasing tree cover in a distributed fashion might help slow the rate of water runoff or increase the rate of carbon storage.
But they aren't going to be ideal from a habitat standpoint. Instead of having 10% of land on each farm converted to trees, it'd maybe be better to have 10% of farms, chosen in contiguous blocks, converted to forest. Like, have the government offer to buy farms adjacent to existing woodland to expand it.
googles
It looks like there's a separate initiative to create contiguous woodland in Wales:
https://en.wikipedia.org/wiki/National_Forest_for_Wales
The National Forest for Wales (Welsh: Coedwig Genedlaethol i Gymru) is a long-term forestation programme by the Welsh Government, aiming to form a network of woodland throughout Wales.[2]