Crown copyright DOES exist outside of the UK, although in slightly
different forms. It exists in Canada, Australia, New Zealand and a
number of other countries. The UK version does exist in name outside
of the UK, although almost certainly the duration of the copyright
would be no different than ordinary authors under the Berne
Convention.
However, given what the OPSI have said in that email we are perfectly
safe in using published UK Crown copyright materials from 1954 or
earlier in the Wikipedia. Regardless of the exact term that the laws
of each country might provide, they are the people that administer UK
Crown copyright. That means they determine how long they will enforce
the copyright outside the UK. If they, as they have, say that
published UK Crown copyright works will be public domain 50 years
after publication worldwide, not just in the UK, then they are
perfectly able to do that.
Well, once again, given the Bern convention any author can enforce
its rights by himself even if his work is public domain or equivalent
in its own country. And the crown copyright office has no authority
on this.
What will they do if such a case happens ? What kind of warranty do
they provide ?