The woman would be early to mid 1880s, correct? It looks like she still has a bustle going on though the hair is a bit tamer…
Not sure what it’s worth, but here’s a link to another example of the same Webster Bros of Bayswater and what their cardstock looked like at some point in the 1870s:
http://www.cartes.freeuk.com/time/back70.htm.
As for the man, I again agree with China, and say this was most likely taken sometime during the 1870s. In America, mutton chops were popular into the 1860s then slowly began to fade out of popularity among young, fashionable men by the 1880s. They were still seen among the older men who had worn them in their youth to the end of the century but I suspect a man as young as this would probably be more up to date than that. Additionally, though cabinet card portraits were introduced to the world in 1866, they are pretty rare ( and probably particularly in America) before 1872. Before then, CDVs, ambrotypes and the ever-popular tintype were more likely. I’m not sure exactly how well this lines up with Cabinet Cards, but I know that in Cartes de Visites, yellow cardstock was popular from about 1869-1874, white card stock was popular from 1858-1874, and rounded corners became popular in 1871. Specifically pertaining to cabinet cards, however, I would say that the design on the back of this mount is pretty indicative of the 1870s as well.
