That depends on what you mean by "definitive"! My house was registered with the Land Registry when I paid off my mortgage in 1994. [For the earlier part of my ownership, and that of the previous owners, it had been unregistered].
The document produced by the Land Registry refers to "The Freehold land shown edged with red on the plan of the above title filed at the Registry . . ". But the plan attached to the document is to a scale of
1:1250 and the red line between my house and next door is almost as wide as the gap between the houses - so you'd be hard put to see exactly where the boundary is. There are no dimensions stated anywhere in the document. I don't know whether they have anything else in the Registry itself.I also happen to have a copy of the original conveyance between the builder and the first owner, and that has a 1:500 plan which states the area of the plot in square yards and the length of the frontage in feet and inches. Even that is subject to *some* interpretation!