Police in general as "feds"
There are many slang terms for the police, and one which has recently been in the news in the UK is "the feds", as in
if you see a brother... SALUT! if you see a fed... SHOOT!
Cassell's Dictionary of Slang (2005) records this sense as "[1990s+] (UK Black/teen) a policeman". It seems likely that this is a straightforward borrowing from US film and television, where the Feds are agents of the Federal Bureau of Investigation; the UK is not federal and does not have an FBI.
The Royal Air Force police are also called "feds", among other nicknames. This usage is in Partridge's Dictionary of Slang and Unconventional English, 2005 edition, but not in the 1984 edition, nor is it in the 1990 edition of Partridge's A dictionary of RAF slang. In the 2005 dictionary, the RAF usage is dated to 2002, with the note "Adopted from the sense 'a member of the FBI'". As far as I can tell, the nickname is not used elsewhere in the UK armed forces.
Are these two independent borrowings, or has there been some additional influence from one to the other? Are there any other similar slang uses of "feds" in the English-speaking world (referring to police or officials who are not associated with the federal government)?
Update: On reflection, I think that they are probably not related. The point is that the urban "fed" is used for any policeman, whereas the RAF "fed" is only for members of the specific force: it's not used within the RAF for police in general.
My second question - whether there are other (probably ironic) slang uses of "fed" for non-federal police or officials - is still open.
Solution 1:
I think it's highly unlikely OP's quoted usage has any significant prevalence. I'm not aware many of the UK's disaffected youth are seriously keen to be seen as "Americanised".
That said, and as we can see from recent events, obviously the situation is quite extreme at the moment. People get confused, they have trouble distinguishing real life from the daytime tv American cop serials they spend their time watching, they forget which country they're living in.
The Guardian was probably happy to report that particular young hothead's words because they reflect the surreal nature of what's happening. I doubt it's an emerging slang use, partly because it would be too prone to ridicule in anything like normal circumstances.
Also note the word "Salut" in the invective (French for "Hello" and/or ignorant spelling of "Salute"). Note that this wasn't recorded "on the street" - it was a shit-stirring message on BlackBerry Messenger. It could have been from a non-native speaker, not necessarily even in the country. Someone working for a hostile foreign military seeking to destabilise the UK, for all I know.
Solution 2:
This was probably borrowed from the USA, where it means "a Federal law enforcement agent".
Over here we have multiple tiers of government, each with their own law-enforcement agencies. The chief agency at the top ("Federal") level is the Federal Bureau of Investigation (F.B.I.). Their law-enforcement agents didn't used to be a big deal, but that branch had to grow drastically in the 1920's when we (foolishly) tried to outlaw Alcoholic beverages.
This era is enshrined in probably hundreds of prohibition-era movies, where the conflict was liquor gangsters on one side, and Federal agents on the other. The stereotypical scene is a secret distribution center being broken into, and the bad guys running for their guns yelling, "It's the feds!". So "feds" over here became a word Federal law enforcement agents.
Proper use of the term would be to refer to only cops working for the country as a whole. (I think your closest equivalent would be foot-agents working for the Serious Orgaised Crime Agency?) However, I could see where it might lose that distinction in translation.
Solution 3:
Another possible influence in popular culture is the use of "Federales", from Mexico.
http://en.wikipedia.org/wiki/Federales