Hi trigpoint.

In England the speed limit laws (Road Traffic Act) apply to all roads that the general public have at least "Tolerated Access" to.

For privately owned driveways and footways leading to a property, it is considered the public have "tolerated access" to reach the front door. This won't be the case if there is a locked gate, a sign specifically revoking access. Just stating "Private Road" is not enough.

But, UK speed limits mostly require signage to enforce, so hard to enforce on many Private Roads.