The OS maps the borders of local authorities. However, it doesn't hold the authoritative map of the borders of the counties. I don't mean the modern local authorities that call themselves counties - I mean the traditional counties, which haven't been abolished or changed for centuries. The exact location of these is defined almost tautologically - a place is part of a certain county because it has always been. The exact location of the border is more a matter of folklore than anything else.
Likewise, the exact location of borders between properties is usually ill-defined. Only borders that have gone through a painstaking process of surveying and registering (usually after a dispute or on a new build) are exactly defined. Usually, when you move into a new house, you have a fence around your property, and the boundary is generally assumed to be inside the fence (with conventions for whose land the fence sits on that aren't always followed).
Adverse possession makes it more complicated. A neighbour decided to put up a new fence, and weren't nice about it. I stipulated (in line with the law) that no part of the new fence could be placed on my property (except parts under the ground surface). They decided to put the fence in the "wrong" way round, with the vertical posts on their property, and the fence attached on their side of the posts, effectively giving me sole access to the area of land in-between their fence posts. If I were to sell my house, the new owner would be quite reasonable in assuming that that land was theirs. After the requisite time, I could apply to have the land legally registered as mine, although the new laws mean that the neighbour would be informed and given a chance to "evict" me from the land - they would have to tear down their fence and rebuild it the right way round so that I don't have sole access to it any more. In this way, adverse possession effectively resets the boundary to where the actual fence is in practice every now and again, which means that the land registry doesn't need to store exact boundary locations.
My parents' house is a good example of this. The rear boundary of the property is defined as being some distance from the road. It's not totally clear (although I assume that there is a standard) whether this means the curtilage or something else. When the land behind the property was redeveloped (from a run down witches cottage to a block of flats) it was important to my parents that the treeline at the rear of the property not be cut down. The deciding factor was the rusted remains of an old fence on the far side of the trees. I had been under instructions as a kid to not mess with that or pull it up because my parents knew that it was important to preserve this kind of evidence
Likewise, the exact location of borders between properties is usually ill-defined. Only borders that have gone through a painstaking process of surveying and registering (usually after a dispute or on a new build) are exactly defined. Usually, when you move into a new house, you have a fence around your property, and the boundary is generally assumed to be inside the fence (with conventions for whose land the fence sits on that aren't always followed).
Adverse possession makes it more complicated. A neighbour decided to put up a new fence, and weren't nice about it. I stipulated (in line with the law) that no part of the new fence could be placed on my property (except parts under the ground surface). They decided to put the fence in the "wrong" way round, with the vertical posts on their property, and the fence attached on their side of the posts, effectively giving me sole access to the area of land in-between their fence posts. If I were to sell my house, the new owner would be quite reasonable in assuming that that land was theirs. After the requisite time, I could apply to have the land legally registered as mine, although the new laws mean that the neighbour would be informed and given a chance to "evict" me from the land - they would have to tear down their fence and rebuild it the right way round so that I don't have sole access to it any more. In this way, adverse possession effectively resets the boundary to where the actual fence is in practice every now and again, which means that the land registry doesn't need to store exact boundary locations.