I would look for a parish church in St. Pancras where the Deas children were christened - son David appears to have been christened in St. Pancras (on the IGI) - so you could try and find the later baptisms and see if she was christened in the same place.
A possiblity is that the father was actually the son David, who was 18 years old in 1871, given the surname 'Willis' on the certificate so that it would match the surname Elizabeth was using. I can't spot son-David in later records - can anybody place him?
(There is a family living at Rochester Terrace in 1881, near to College Street I think, headed by Henry Willis, organ builder, daughter Edith, 30, apparently did not marry and is still in Pancras in 1901, as a pianoforte teacher. They are also there in 1871).
Janet Deas probably died in 1908, aged 83 - might be worth looking to see if she left a will.