According to the "Excalibur" commentaries, Helen and Nicol had just ended a disastrous relationship, culminating in a disastrous run of 'The Scottish Play' and swore they would never work together again.
When asked who she would like to see play Merlin, Helen supposedly replied, "Anyone but Nicol Williamson!"
When asked who he would like to see play Morganna, Nicol supposedly replied, "Anyone but Helen Mirren!"
I couldn't see any two performers in either role in that movie. I paid to see it in the theater over 20 times.