LOL, no that's simply not true.

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.

Most of the Scottish border is fairly physically obvious, unlike Wales (although the Severn bridge is).

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.