I don’t think there is a simple black and white answer here. If the wide field margin is truly for wildlife, walkers, particularly those with dogs, may disturb the wildlife and negate its positive impact.
I am not of the mind set that I want to walk somewhere and will do so whether I am on private land or not. I do agree that more access to open country is required, but I am not sure that trespass is the way to go. Perhaps lobbying for a change in the status of lost ways or for more open access, as we have in Scotland, would be a better option. We could definitely make government grants for land management dependent upon opening up access. Unlike some others on here, I don’t believe the size of the land someone owns should necessarily affect whether the public have the right to walk on private property. That smacks more of jealousy to me (I don’t own acres of land by the way). I did know someone who took part in the Kinder Trespass in the 1930s - he was an amazing guy who used to cycle from Manchester to Snowdonia on a Friday evening for a weekend of climbing before cycling back. He was one of the least confrontational people you could ever meet.
We need to mobilise a popular campaign to establish a new paradigm and try to bring all stakeholders along, rather than promote confrontation.