This is how it was explained to me. Assuming he doesn't own the lane, and it may not matter in either case.
Say you agreed to buy 1 acre of land from him.
You agreed to buy land, not lane.
The acre of land is measured to the boundary of his land which as I understand it is to the centre of the ditch.
When the site is registered with the land registry, the site boundary will be extended out to the centre of the lane. This is a more reliable reference point over time than a ditch which could be moved or removed. The lane is alot less likely to move.
The map with the land registry will then show more than 1 acre i.e. your acre plus the area between your site boundary and the centre of the lane.
We purchased a half acre with OPP. The site boundaries as submitted with the planning application did enclose a half acre which was to the centre of the ditch. I have seen the land registry map however and the size is marked as 0.62 acres and does extend to the centre of the lane.
I am open to correction however when you find a definitive source.