Okay so I've thought about this a bit more and come prepared with diagrams
Here's west horsley place (where they filmed ghosts) in real life. I've crossed out what is apparently an opera house in the north and decided that that's all woods in the ghost universe. The red dots are approximate death locations of the ghosts (we don't know where robin died). The blue lines are my headcanon for where the property boundaries lie. It's about 350m from the house to the gate house.
Here's a zoomed out picture based on julian's radius being about 350m. If we take that for the other ghosts we can see that thomas has access to way more westerly fields than anyone else. (red is house ghosts, yellow is mary, beige is pat (for his shirt and shoes and socks))
I like the radius theory too but it's self evidently not true when you consider the badger trap ghost
This one has the most evidence for it imo. With the modern land registery it's much easier to define what counts as part of an estate than in previous times. However a national land registery's only been around since 1867, so what about the older ghosts?
For Humphrey I think he originally had access to all of the button house estate, even though he was renting parts of the estate out to his tenants. This would mean that the common land didn't pose problems for ghost boundaries and that he probably had a larger area to roam orginally. The reason why I think this is that the plague ghosts are all together in the basement, even though they probably died in seperate houses. (though the plague ghosts are an outlier anyway because they seem to be connected to their bones?)
Robin is a different matter. As far as I can tell, formal land ownership was brought to britain by the romans, and the Ghost's wiki says he was born in approx 10000BC (but then he also says he's 10000 years old which would be more in keeping with mesolithic occupation of Great Britain). He would have been at least semi nomadic and it's impossible to know what mesolithic britains considered land ownership. As Britain was at this point connected to europe, on the extreme "no land ownership" end of the scale, he might have been able to roam through most of eurasia. Based on pure speculation I would assume that there's either a natural barrier to prehistoric ghosts eg geology or biome related or that his tribe had a communal area that they considered theirs. Either way he has been increasingly confined by subdivision for 10,000 years
3. Those bastard Nazi ghosts
These guys (who according to the wiki are called helmut and wolfgang) are the main arguement against the property title theory. According to English law, your property includes the airspace above it and these guys weren't that high up as they were seen from a window. They should be wandering inside the flat alison viewed them from.
a. Vehicles count as seperate property. Easily debunked as Pat died in a bus
b. The plane somehow counted as Nazi Germany, like with embassies. That may have been the case in the 1940s, but the owners of the property have changed since then.
c. Some deaths are tied to items rather than places. This would also explain the plague ghosts and why they like to be so close to their skeletons. However you would expect the Nazi ghosts to be haunted the scrap from the plane rather than just the air.
d. The universe or the ghost authority or the general psychic vibes of the British people decided to curse them because they were Nazis.
Having thought about it, I woud assume that every named road counts as a seperate road, as they're probably all seperated like that on a database somewhere.
This means the best road to die on is the A1
If you want the biggest area, you might think that a national park is the way to go. However in the uk most national park land is owned privately so if we're going with the property titles theory, don't bother.
The answer is probably a bigger stately home. A rough google search has uncovered places like Wentworth Woodhouse or Elveden Hall as good candidates for sheer amounts of space and of course there'd be more ghosts to talk to if Button House if representative.
If you want to increase your odds of being seen by a psychic visitor, you should die at the Tower of London, one of the uks most visited castles.
I think my preferred theory is that ghosts are confined by what the majority of people think the boundaries of the property are, rather than what's written on a bit of paper somewhere. That would make more sense for older ghosts as it would be a bit shit if they had continents to roam across and that got destroyed. It is still very funny for whatever causes ghosts to be sitting in an office pouring over titles and deeds.
Also Robin has a right to be salty