There are also a number of cairns marking the Anglo-Scottish border,
which also denoted sites where the English and Scottish local
overlords would meet periodically to sort out disputes (the Borders
being a notoriously lawless area before the Union of the Crowns in
1603).

Incidentally, it is (or was) also possible to walk between UK and
Norwegian jurisdictions in the North Sea on an oil platform complex
that straddled the UK Continental Shelf/Norwegian Continental Shelf
boundary. Although not strictly an international boundary, as
(certainly) UK platforms were not regarded as part of the UK proper,
though subject to UK law in most areas, it would be interesting to
know if the divide was marked on the walkway connecting the UK and
Norwegian platforms.

It may have been the Frigg Field complex - I will try and remember
(or better still, look it up).

This is an excellent and very interesting site - thank you all very
much.