An email from a friend in marketing (german products to UK consumers) asked about teenage boy's language in London:

I have a small but timely favour to ask (I need to think of something by tomorrow pm). I need to basically say the following phrase 'for sharing or just for you' but talking to 14 year old boys. How do they speak?? How would you give them this message so they wouldn't think you were an idiot, but also sort of the right language?

Firstly, 14 year old boys are hugely competitive - the message would go down better if you played on the idea of not sharing. They're interested in cars, social success, football, and playstation/xbox games on rallying or war strategy games. I can't underestimate how important football and playstation are.

Having the right mobile with downloadable stuff is dead important, too, but mostly to impress girls. They pretend to be interested in girls, but aren't really up to the challenge yet. They're more motivated by getting one over on their mates.

The sort of thing they read is Sky Sports and Gaming Cheats sites, as well as the gruesome bits of the Guinness Book of Records.
Things that are funny or unexpectedly outrageous are hugely popular.

You haven't said what it is that is for sharing - an object? A food? Again, I think saying that something's too good for sharing is the best way to get this lot interested. They don't know the intrinsic value of things, and look to peers to validate whether something's good.

As for slang, words of which a teacher is aware are probably out of date already.

Something good is 'raw', or possibly 'dark', though not always. 'Wicked' still exists, but as an intensifier only, not as an adjective.
In London, it might be 'true-true', but that's probably only in heavily integrated afro Caribbean cultures. I think 'da bomb' is possibly too female.

'Blatantly' is a good superlative. If you're good at something, you have 'mad skills'. A party that's good is 'banging'. Your friends are your 'crew'. Girls are 'fit', unless they're known to be easy/sexually active, then they're 'slack'. If you like a girl and chat to her, you're 'sharking' her.

The use of slang is a way of circumventing society's taboos, and children's choice of slang phrases reflects their concerns from within a closed environment.
Threat is perceptibly a large influence on young men's lives. Bullying someone is 'taxing', thieving is 'tiefing' someone's things. Insulting them is 'cussing'. Physically intimidating someone through closeness is 'fronting' or 'getting in yo face', rather than actual violence, which is when you 'bust up' or 'bang up' the victim.

When you contradict someone, you say 'nahman, nahman'. Someone who's casual about authority or lives outside the law you'd describe as 'breezin', or 'breezin through', right until the law catches up with them and 'caps' them. Someone who's a nerd is a 'boffin'.

This dictionary of teen slang is pretty accurate, though slightly Americanised for the children I teach:

From the list there, I've heard boys use: 24/7, all that, fool, busted, fine, homeboy (yeah, really!), ho, kicking, keep it real, skanky, phat, rude bwoy (not rude boy), skater, slacker, tight (as in loyal), tripping (as in imagining things or over reacting), whatever.

'Butters' in England means someone deeply ugly with a nice body, though, not the positive spin it has in the US.
In fact, most London kids take their lingo from Jamaican culture, not so much American: therefore words like 'rass' or 'bambaclart' are beyond the pale swearing in a way that US teens probably wouldn't recognise. Likewise, 'batty' is a widely used phrase (as in 'battyman' or homosexual - or actually, 'batty riders' are just tiny girls shorts), and to call something 'gay' means it's stupid or it doesn't work, rather than sexualising it. 'Innit' and 'ain't' endure horribly, and a majority of London kids will have a sprinkling of understanding of cockney rhyming slang, though actual use of it varies widely, and is rather an antique, parentally influenced idiom.

Urban Dictionary is a brilliant slang info site, but is probably more adult, and more sexual than fourteen year olds might be familiar with.
Other decent sources are the Peevish dictionary of British slang, Wikipedia's listings of hip hop slang.

Of course, move twelve miles down the road, and it'd all be different again.