A British artist dies and the 70 years pma rule comes into effect. However, his daughter sits on the board for the artist's foundation and does not want copyright to expire after 70 years. Can she renew copyright for his works somehow and if so, what are the limitations on this?
The duration of copyright is specified in the Copyright Designs and Patents Act 1988:
12 (2) Copyright expires at the end of the period of 70 years from the end of the calendar year in which the author dies
The only provision for extending the copyright is where is some doubt about authorship. If an unknown author is identified within that 70-year period, then the copyright expires seventy years after the death of the last-surviving author (ibid.)
In the case of an artistic work which is presumably verified as being the work of the artist and protected as such, it would be very difficult to suddenly "discover" an unknown contributor who would cause the copyright to be extended.
The copyright for the original work will expire. That foundation is free to create copies of the work that are slightly modified and claim copyright on the modified work. Now I would personally not be happy to buy a modified copy of a great work on art, so I doubt that strategy would be too successful.