The entire purpose of Orwell's essay is to prove the degradation of both political speech and the English language. In paragraphs 15 and 16, Orwell is using metaphor and simile to indicate over-reliance on tired-out, radically over-used phrases. The purpose of comparisons as found in similes and metaphors is to help readers associate the potentially unknown with the known. The simile or metaphor is generally known, and the author is introducing a new known fact, or is hyperbolizing a little-noticed known fact.
In the first metaphor, the "packet of aspirins always at one's elbow" Orwell uses this idea of a readily attainable pain reliever to describe the effort writers go to in order to make a point. This is ironic it suggests the pain of writing well. Authors will take the easy way out to remove the pain by relying on an over-used phrase. Furthermore, the proximity of a person's elbow is ALWAYS within range of the person. Orwell is suggesting that there is no reason to search the depth of one's soul, a thesaurus, or a dictionary to find just the right metaphor or slice of color to clearly and cleverly craft a metaphor.
In the second, "cavalry horses answering the bugle," readers find Orwell using a comparison that communicates consistency and absolute assurance, although there is not a positive connotation to be found. Orwell is expressing the author's words to which he refers are absolutely predictable, just like the answer to a bugle cavalry horses will provide.
Finally, his "in the job" expression demonstrates the need for people to dedicate themselves to improving language. This comparison of how people treat the process of writing to how people address their jobs that they get paid for demonstrates Orwell's overall message. It takes work to cleverly craft articulate speech or writing. It will take the determination that is demonstrated by employees who work for 8 hours a day in order to earn a wage.
All of Orwell's similes and metaphors that you have cited here have original meaning. They also serve to further illustrate his point that language is becoming basic as people choose not to work hard to communicate with clarity or beauty.