You agree the niqab doesn't send a positive message.
This explains the negative reaction.
Yes, I agree that women should not be judged on what they choose to wear.
But, Khadija's choice is based on an ancient patriarchal religion which often enforces 'modest' clothing, and imprisons and silences women. And only women, not men.
Khadija may well see her niqab as an expression of her choice and freedom, but her freedom may be the result of the privilege of living in the UK instead of in, say, Afghanistan. Many women do not have her freedom.
She can do as she likes, of course, because in this country she is free to live as she wishes, but the negative reaction cannot come as a surprise in a country in which the vast majority do not adhere to the idea of women shrouding their entire body except for a slot for their eyes. To us, it looks freakish.