In common law a highway is “a way over which there exists a public right of passage, that is to say a right for all of Her Majesty’s subjects at all seasons of the year freely and at their will to pass and re-pass without let or hindrance”
A verge would be part of the highway as someone could pass and re-pass without hindrance (e.g. on a horse).
County Councils are required to keep a definitive map showing all footpaths, byways and roads used as public paths that, in the opinion of the County Council, are public rights of way.
If a highway is shown on the definitive map it serves as conclusive evidence of its status as public highway and it is a criminal offence for any person, without lawful authority, to obstruct it.
Ditches along side a road are considered to be an obstacle to passage, rather than something over which one can can pass and re-pass. The initial presumption is therefore that ditches are not part of the highway unless it was constructed in order to drain the highway.