Ms. Emma Thompson (lovely lady) has had a go at the way young people talk, something she calls 'slang'.
"Just don't do it ... because it makes you sound stupid ... We have to reinvest, I think, in the idea of articulacy as a form of personal human freedom and power."

I have to agree with the lady. Most 'snotnoses' (J.H. slang for the young) get on my wick when they come out with the same old tired words, 'wicked', 'minging'. 'Bling', like 'cool' is now an essential part of middle class vocabulary, and should be abandoned by any self respecting laicophone*. As the old Readers' Digest used to preach, it pays to increase your wordpower. My ten-year old granddaughter says "wicked", which, to me, means that the word is not slang but baby talk.
Ms. Thompson, it seems, is not referring in the main to slang , but to the inability of some to form a sentence without resorting to the the words 'like' and 'innit'. Robert McCrum seems to be alluding to this lazy terminology when he calls slang "sloppy language". Real slang is no such thing.
G.K. Chesterton claimed that all slang is metaphor and all metaphor is poetry. Far too sweeping a statement. Metaphor has part to play in slang but it's not the whole story.
I don't know when I became interested in the subject of unconventional language, but it must have been at an early age. I remember hearing the word 'palooka' in an American film, liking the sound, wondering what it meant, and how it came into being. I knew instinctively that it wasn't a regular dictionary word.
At school I was introduced to a slang, perhaps the proper term is cant, spoken by some of my schoolmates. It was language that nobody else spoke and for a long time I thought it the monopoly of my particular sub-culture (the poor kids of Hull). After school I found that some adults spoke it. Then, when serving queen and country I learned that it was known, with local variations, in other parts of Yorkshire.
Much of this nameless tongue was a debased version of Romany, words like -

Cushty - 'good' (or general approval)
Chavvy - 'child'
Nash - 'go' (in Romany meaning 'run away'?)
Kenner - 'house'
Peeve ken 'pub'.

Other words that may have been Romany were -
Musker - 'police officer'
Mingra - (same meaning)
Mang - 'speak, talk, say, tell'
Looer - money
Divvy - 'daft', not right in the head (sometimes "divvy in the mundy" [?])

Then there were other words, such as -
Bewer - woman (or girl, but not pre-puberty)
Scran - 'food'
gage - 'pint' (spelt gauge?)
Noggins - 'testicles' ("Kick him in the noggins!")

To this vocabulary, not universally recognised, therefore a true slang, I added other languages over time. Rhyming slang -
Jockeys' whips - 'chips'
Holy ghost - 'toast'
Joe Baksi - 'taxi'
Irish rose - 'nose'
Napper Tandy - 'brandy' (also 'Mahatma Gandhi')

Then there was the army slang, much of it not transferable to Civvy Street; fizzer, monkeys (MPs), jankers. In North Africa, some Arabic and Italian was added to the mix -
Shoofti - 'look' see'
Imshi - 'clear off!'
Floos - money (mafeesh floos, 'no money')
Bint - 'female' (all ages)
Quanta costa? - 'how much? (Italian, recte, quanto)
plus some strange lingua franca from round the Mediterranean and further afield -
Chico - 'child'
Mongheria - 'food'
Klefti-wallah - thief
Queenie-wallah - homosexual.

And many more sources to enrich my vocabulary and add to the unintelligibility for which I'm noted in some circles. This subject fascinates me, if no-one else, and shows that all slang is not stupid or sloppy. I may return to it sometime.

*A word have just coined, which may turn out to be meaningless. It's supposed to mean a speaker of true slang.