FYI I’m continuing to research this.
Some of the parishes within England (eg Brighton) share kml files of all the council-owned agricultural land, which we may be able to use as a starting point. I’m not sure how receptive the council will be!
I’ve also found that here in the UK there was an extensive survey of all farmland in 1943. I may shell out to get the details about the land that I am purchasing now - I’m not sure how much it has been subdivided over the years, but I’m on friendly terms with most of the neighbours. This could be a nice way to get some boundaries that are realistic without being relevant.
Other software sites do have a caveat in their videos / screenshots / etc. that this is example data, not a real farm.