The borders between Scotland, England, and Wales are effectively like state borders in the US. Unless you see a street sign, you wouldn't know you crossed it.
NI is physically on a different island, so requires a boat or plane ride.
And all of the UK (ignoring the territories and ex-colonies) is on islands. And require a valid passport, passing through customs/immigration checks, and either a plane, boat, or train ride.
Tens of thousands of people live on islands other than Britain and Ireland. Much of Scotland is not even considered UK mainland by delivery firms, even major places such as Perth.
I can understand Lewis and Harris or the Shetlands (they're separate islands without bridge links), but Perth doesn't make much sense. And I would have assumed Skye was considered "mainland" as well, at least now that there's a bridge.
There was a controversy about Perth in this regard a while back. I'm not even talking about Blairgowrie or Dunkeld or outlying places but someone trying to get deliveries to an industrial estate within the city of Perth itself.
The way things are going, maybe only the Central Belt will get preferential delivery fees.