Hi gurglypipe, 

We can turn this around and ask "what is the source for tagging these ways as  access=private

For OSM access=private means that journeys along the way (or through a barrier) require specific permission. Therefore the access=private tag would stop necessary journeys to a front door (eg Post & deliveries)

In the UK it is accepted that there is a need to visit the front door or main entrance of a property on private land. Therefore the UK has an "Implied Rights of Access" to reach a Front Door or Main entrance. This  "Implied Rights of Access", must be used in a minimal way. That means pedestrian only, unless the journey involves a long journey to a front door (eg farm), which case use of a vehicle is considered reasonable. 

The most reasonable tag we have for this is access=destination for the way that is used to reach a front door.