Where do the mother and her son want to go?

A mother and her young son get into a bus in a small city and sit down. The bus conductor comes to them for their money. The mother says, "I want one ticket to Oxford," and gives him a shilling (先令).
    The conductor looks at the small boy for a few seconds and then says to him, "How old are you, young man?" The mother begins speaking, but the conductor stops her, and the boy says, "I am four years old at home, and two and a half in buses and trains."
    The mother takes six pence out of her bag and gives it to the conductor. The conductor gives her one and a haft tickets.