Boundary title is sometimes complicated and unclear – the boundary lines on the land registry maps used can be up to 11 meters out, legally. It’s very common for landowners not to know who owns a hedge themselves, not every boundary line has the lines indicating ownership.
Roadsides are generally easier though – typically the landowner owns the land to the centre of the road, but not the road itself. I’m a little fuzzy on the road part, but fairly clear that hedges are usually owned by the field owner behind them, and upkeep is their responsibility; hedge trimming is usually done by that farmer or local farmers pool resources and share the job.
Sometimes though, the council pays for a specialist to do hedge cutting too – especially on major routes.
But don’t tell the reviewers, they’ll probably ban every cache within 100 meters of a road…