Earlier quoted context omitted.
I don't know if borders matter much in this case. It's approximately just as easy to travel to England from many other countries as it is to travel to England from the rest of the UK.
It is a lot harder to travel into England from outside of the United Kingdom than it is to travel from England to Scotland or Wales. For one, I do not need a passport to get on a train to Scotland. Nor do I need a visa.
If you want to get to Glasgow from Campbelltown it can take you as long as three hours for example, and that's before you hit the motorway. Trying to get from west Sutherland to London is worse. Trying to get there from the Hebrides or Northen Isles even more so.