Rapunzel, many years ago I used to be "the woman who joined in with my husband when he sang" and rarely got asked to sing on my own at festival singarounds. The person who broke the mould was Tom Wilson, who said (after my husband had sung) "well, that was him, now what about one from you" (or words to that effect). It was a real turning point for me. That was about 1980, and as you say, unforgiveable now. Good for you for making things clear to the organiser, and happy singing wherever you are.
Saro
PS What is happening in Oxford to cause all your great singers to flee the country??? The world needs to know...