I do have a birth certificate in which a mother's previous marriage is not mentioned (she married at 18, her first husband died young, no children to the marriage, she then remarried at 20 under the first husband's surname).
I also have known people to have lots of certificates swearing the mother's name was, say, Mary Smith formerly Brown, but no marriage found until several years after the birth of the children.
Of course, there's nothing to say that they married in Pancras, just because the child was born there. On the original birth certificate (presumably in 1867?), what was Alexander's occupation, the address of birth, the address of the informant (possibly the same), etc? Was Emma the informant?