"we took the Underground to Fenchurch Street"
This is a curious line, as Fenchurch St railway station
is not part of London's Underground network. It is not clear whether this is a mistake on Stoker's part, or a joke, or whether he is deliberately reshaping London to give it a fantastical feel, in the same way that Philip Pullman takes liberties with Oxford in His Dark Materials
To reach Fenchurch St via the Underground, they would have travelled to Mark Lane tube station on the Circle line, which opened in 1884 and closed in 1967.