“demonstration” in this sense is synonymous with “protest” (The verb form is “demonstrate.” and people who demonstrate are “demonstrators.”) The anti-nuclear demonstrators referred to in the text were demonstrating against the use of nuclear power as a source of energy because, as they believe, it is too dangerous.
if you are “enthusiastic” about something you really want to do it, find it very interesting, think it is a good idea etc
natural to imagine:
“natural” here means “normal,” “what is to be expected.” “Imagine” when it is used in this way has a meaning very similar to “believe” or “think.” (There is no suggestion that the thing being imagined is unreal or merely mental.)
A scholar (usually a university professor) is someone who is studying something (usually a social, not a technical, subject) at a very high level. (The word is cognate with “school” and in the past was used to refer to any student.)
public order policing:
“public order” refers to the normal, calm operation of society; when this is disturbed — for example, by violence in the streets — public order is lost and must be restored by “public order policing.” “Policing” is the -ing form of the verb “to police,” which can be used to refer to “police action.” (This verb is not common and is quite formal.)
someone you work with is your colleague. The word is most often used to refer to someone you work with in your “profession” (law, medicine, architecture etc) or your business.
if something written or spoken in one language is put into another language it is a translation. (Or, to use the verb form “translate,” it has been “translated.”)
have lost affection:
if you have affection for something, you like (or love) it; if you later “lose affection” for it, you don’t have that feeling anymore.
“Presumably” is an adverb which allows the speaker or writer to express some doubt about the statement being made. It has roughly the same effect as beginning the sentence with “I believe that” or “it seems” (“Presumably” belongs to a class of adverbs called “content disjuncts” see: A Comprehensive Grammar of the English Language, Quirk, et al, 8.127.)
“metaphorical” is the adjective form of “metaphor.” A metaphor is a word or phrase that works not by saying what something really is but what it is like or similar to. The use of the word kettle to refer to the area inside the police cordon is metaphorical because that space is not really a kettle but is like one.
“dignified” is the adjective form of the noun “dignity.” A dignified person is one who acts, looks, thinks, feels in a way that makes him or her worthy of respect.
in this context it means more or less the same thing as “common.” To say the idea is popular just means that a lot of people have it; in other contexts, e.g. “Jack is a popular teacher,” it means something like “well-liked.”
a fanciful idea is one that is not true, or at least one that is not based on sold reasoning or evidence.
To say that “kettling” derives from the Latin word catillus is to say that it comes from (or “originates”) in that word.
“opponent” is the noun form of the verb “to oppose” meaning “to fight against, to try to destroy or defeat.”
if you provoke someone you cause them, by your words or actions, to do or feel something (often a strong negative feeling like anger or aggressive action). The quite common adjective form is “provocative.”