If it's news imagery, as above. However, a great deal of breaking news imagery is just crowd sourced now and is basically just lifted from twitter etc. Long gone are the days where a photographer is dispatched to an unfolding news story in the first instance.
What Pete says is only true for news imagery though. Editorial, fashion, advertising.. are all still often commissioned, so in effect, yes, they will give you a "job" but only on an assignment by assignment basis.... there are no "jobs" in photography in the conventional sense.
Many publications will feature your work if you send it in on spec though, but you're very unlikely to get paid. Arts and Culture type publications often feature new or interesting artists for instance, so it's not impossible to get featured somewhere. This is a good way to get known actually, but you need to be producing work that is worthwhile for them to publish... interesting work. If it's just the usual amateur fare... forget it.. no one's interested.
Editorial, advertising, fashion etc? If you need to ask, then you've no chance unless your images are phenomenal as you'll be too naive to cope with that ****storm

. The best way in is still assisting. Self-publishing is another way to get your work out there and seen. You can send a folio to an agent to see if they are willing to represent you as well.
You can send a folio direct to the picture editors of magazines, but you can imagine how many people do that, so don't hold your breath.
What are you doing to get your work seen by the people you want to work for? If the answer is nothing, then it will never happen. You need to get your work published as much as possible.. get it out there and seen by as many of the type of person you need to see it. Get it featured on websites etc. Obviously, to do that, it needs to be good. Look for artist/photography galleries online, art collectives, magazines etc. that feature the kind of stuff you do, and send them work.
Also... are you any good? Competition is stiff and most publications want something that gives them an edge. What do you shoot, and how do you shoot it? You have to know the markets you're trying to penetrate inside out and have work that is suited to it.
As you are referring to London, it makes it even more difficult, as it's a saturated market, and unless you hadn't noticed... everyone's a photographer these days

Plus, there are hundreds of graduates every year flocking down to London with folios full of work, some of it incredibly good. Not only that, if the courses they've been on are good, they'll be clever, clued up, trained to talk about their work and pitch for work in a way you haven't, and they'll know the ins and outs of the industry already to some extent. Most will have already been networking and have contacts before they graduate too.
I'm not saying give up.... but I think you've massively under estimated how difficult it is.
Do you have a website or other online portfolio? If I knew what you shot I may be able to be more specific. I have no idea how good you are. You could be amazing for all I know, in which case, it's obviously going to be so much easier for you and all you'd have to do is get the work seen and then you're on your way.
Got a link to your work?