But they are not legal in this country. See my earlier post.
I cannot see anything in your previous post which says 'A'-frames are not legal in 'England and Wales', assuming that is what you are referring to by 'this country', since you state you live in N. Wales. A principle of English Law is that anything is legal unless forbidden by Statute or under a decison of a Court of Law which bids a lower Court. i do not see anything in your previous post indicating to anything making 'A;0frames illegal, providing they adhere to braking, lighting and signage regulations.
As for the assertion by the Dft, if they did make that asssertion, about non-application of the 'Vienna Convention' to subsequent rules, applying that thinking would make all Construction and Use legislation void across boundaries, equally Driving Licences issued under legislation after 1948.