"Taxi to London" is the name of a short film, which documents, or reports, or tells about two people (travellers) who travel from Asia to Europe.
"...a documentary following two travellers'..." means, the camera staff follow the two travellers around Asia and Europe while filming them as they travel from one country to another.
Subject = "Taxi to London"
Linking verb = is
Object = documentary following two travellers's journey from Asia to Europe
Participle phrase = following two travellers's journey from Asia to Europe (modifies 'documentary'; function = adjectival phrase)
Present participle = following (~that follows)
Object = two traveller's journey [adjective+possessive noun]+noun (object of 'following')
Prepositional phrase = from Asia to Europe (modifies 'journey')