Imagine coming up with a significant amount of money for a painting only to be thwarted from buying it at the last minute by the machinations of the British government. How this is fair, I do not know; the cultural treasures of certain nation/states are, understandably, protected.
The problem here is that Portrait of Mademoiselle Claus is not a cultural treasure of England in any sense of the word or idea; it is a treasure that has been rarely seen and has nothing to do with England at all. It is, however, valuable and that may explain why this is less about art, more about money.
How this stands up in the court of international opinion is unknown. I would think that this would lead to people being very wary of buying anything owned by a British subject and would make it exceedingly difficult for a British subject to sell anything valuable.