It’s actually not law, just custom. Most/all speedometers over estimate for this reason.

The motorway cameras, near Birmingham have been known to issue tickets for doing 71mph.

I would be wary of those roads. I’ve ran across several that seem like national, or 50 roads, yet limited a lot lower. Generally, there is a hidden danger on that stretch. The classic being a blind junction joining, or a school kicking out nearby. It won’t be obvious, unless you are familiar with the area.

At the same time, i also know of a 30 limit on an otherwise national road. It’s along the stretch in front of a previous Mayer’s house.