Thanks for your reply.
In order to be successful in any adverse possession application, this party would have to show that the land is possessed by him, to the exclusion of others, and that in effect, he is using the land as if it was his own.
If third parties were crossing the land very infrequently (say once or twice a year), this would not necessarily harm the applicant's case, but if people are using the land on a daily or indeed only a weekly basis, there is no way this party would be successful in any adverse possession application, after the set period of time has elapsed (10 years if the land is alreafdy registered at the Land Registry or 12 years if it is not registered).
In your example of Person B, this would be just one instance of evidence that exclusive possession has not been achieved.
As rgeards the definition of "factual possession" I can do no more than quote the following, which is the authority on such matters-
In Powell v McFarlane ((1979) 38 P & CR 452, approved in the House of Lords in J A Pye (Oxford) Ltd v Graham  UKHL 30) Slade J said:
“Factual possession signifies an appropriate degree of physical control. It must be a single and [exclusive] possession, though there can be a single possession exercised on behalf of several persons jointly. Thus an owner of land and a person intruding on that land without his consent cannot both be in possession of the land at the same time. The question what acts constitute a sufficient degree of exclusive physical control must depend on the circumstances, in particular the nature of the land and the manner in which land of that nature is commonly used or enjoyed. Everything must depend on the particular circumstances, but broadly, I think what must be shown as constituting factual possession is that the alleged possessor has been dealing with the land in question as an occupying owner might have been expected to deal with it and that no one else has done so.”
In any application for possessory title, the Land Registry Surveyor will also inspect the site, and if it is clear that it is mereluy a wood, which the applicant has tried unsuceesfully to fence in, it will be clear to the Surveyor that the applicvant has not had exclusive or factual possession.
I hope this assists and sets out the legal position.
If so, I would be grateful if you could rate my answer.