I don't understand the word order requirement here. I translated the English sentence as "Ni veturos iel trans Londonon," which was marked wrong. Apparently "iel" must precede the verb if you use "veturos," but it can (or must?) follow the verb if you use "vojaĝos." Why?
Just a matter of missing alternatives in the list of sentences that Duolingo accepts, I would say -- not a grammatical requirement that you have unwittingly disregarded.