The native population of wolves on the British isles were driven to extinction in the 17th century. So they wouldn't have been wandering the streets or the countryside.
The Zoological Society of London is a recent thing in the 19th century. Up until 1836 the Tower of London served as the primary zoo. It was the escape of a wolf and later a monkey attack which led to it being closed and the Zoological Society of London being granted possession of a park and all the animals from the Tower of London. So animal escapes are not unheard of in 19th century London.
This season summary says this in regard to the Resurrection episode.
Ethan does not mind the vampire-summoned wolves.
They are supposing that when the group entered the zoo looking for the vampire Mina Murray that she was somehow able to send the wolves after them. I'm not sure that she was actual able to summon them, as that is not a power listed on the fan wiki.
That being said, if Mina Murray was hiding out in the zoo and knew that the group was looking for her, it was certainly within her means to release the wolves from their enclosure, thus allowing them to prowl the zoo and happen upon the group.
There is also the possibility that wolves are attracted to werewolves, and when they sensed Ethan nearby, they were compelled to escape their enclosure to seek him out.