A picture / example would help here ! There are a lot of inappropriate trees in a lot of places. Also, the size of trees not always matches reality. In some large cities there are (too many) trees in the streets…
When a scenery is properly mapped, you will have areas where trees are allowed (generated) and areas where they are not allowed. Tree density and tree size can be set, MSFS will show it. But that is all expensive handwork, it is too much work to do it everywhere. I noticed some places in Amsterdam have trees very similar to the trees in reality. Beautiful. But that is all handcraft, they cannot do that for the planet…
In the AI composed scenery elsewhere, MSFS shows trees based on e.g. the count of dark green dots on aerials and satellite images. The counting decides… and it can be corrected by programmers for large regions. I suggest you put a few pictures in the reality-MSFS compare topic, or a Zendesk ticket about UK’s countryside. You are the expert, they are not… Programmers can change AI conditions for trees. I don’t know details how they correct things, but it could all be server-side work: these conditions may be set live in a database somewhere, not requiring any new updates.