The only problem I have with that is that many areas that are grassland in the UK now may have been heavily forested and were denuded of trees prior to or during Roman colonization.
Think about it, this is in Britain right? Grassland that has never been ploughed is going to be pretty rare, probably some kind of old ecology that shouldn't be converted to forest willy-nilly. That's my guess anyway.
Presumably because the Trust are tasked with conservation of nature, they cannot recommend converting one land use to another, because that would run contrary to their task.
Can anyone shed some light on the reason behind not planting in open grassland?