That may not be true under British Copyright law and I expect most of those portraits are from British museums or galleries.The National Portrait gallery claimed this in a letter to Wikipedia.They and other British galleries maintain this stand though it hasn't been tested in court. However even an exchange of legal letters as happened with Wikipedia could be expensive.
https://en.m.wikipedia.org/wiki/National_Portrait_Gallery_and_Wikimedia_Foundation_copyright_dispute
One result of their exchange though was that the National Portrait Gallery agreed to licence a large number of low resolution images of portraits to be used (with the correct creative commons attribution .)e.g.Heres one Tudor lady
Eg
http://www.npg.org.uk/collections/search/portrait/mw69849/Anne-Lady-Pope-with-her-children?locid=4&rNo=14 To use click on use this image .
This restriction often applies to documents as well. Images of Census forms are copyright and the images licensed to some pay to view sites.
I pay 8 pounds at my local archives for a photo permit for the day, I can take as many photos of old records or pictures as I wish but I have to sign to say that I won't share copies or publish them on a website without permission in writing. The obvious reason being is that many of their more 'popular documents have been /are being digitised and sold to Ancestry: they have a commercial value.
