One thing I'd like to mention if you're looking at the picture for its cash value. As time passes, the value, like great works by artists, goes up. Paintings sometimes double in value after the artist dies. With a picture like you have that has been authenticated, with the Queen being such an iconic figure in our world, after her death, I imagine that the value of the picture would increase.
I'd hang on to it for quite a while. Even maybe as a family heirloom.