A jobsworth is a person who uses their job description in a deliberately uncooperative way, or who seemingly delights in acting in an obstructive or unhelpful manner. The term can also be applied to those who uphold petty rules even at the expense of humanity or common sense.
"Jobsworth" is a British colloquial word derived from the phrase "I can't do that, it's more than my job's worth", meaning taking the initiative and performing an action that is beyond what the person feels is in their job description. The Oxford English Dictionary defines it as "A person in authority (esp. a minor official) who insists on adhering to rules and regulations or bureaucratic procedures even at the expense of common sense." Jonathon Green similarly defines "jobsworth" as "a minor factotum whose only status comes from enforcing otherwise petty regulations".
I ran across it this morning while ('whilst", since we're speaking British here) reading a story about a meter maid giving out tickets. I've been calling people like this "tin badge bullies" since I was in 6th grade, and noticed the instant transformation of some of my peers who, elevated to the position of "safety patrol" suddenly became petty little dictators, governing behavior on stairs and sidewalks. "Jobsworth" 's better.