[@Frettzo] I think I'm the only Brit here.

London is generally the go-to place for newcomers. It is, quite possibly, the most cosmopolitan city in the world.
The atmosphere isn't great anywhere for immigrants, refugees, and minorities at the moment though. It'll be hard getting in, and it'll be hard to stay, as the government is really cracking down on the numbers who are allowed in. And you've got right-wingers doing their thing.

As my only real experience is here in London, I can only tell you what it's like here. If you have any specific questions about London, feel free to ask and I'll do my best to answer.

As for how long it would take to find a job like what you say, it's hard to say. I can't really help you there. I know the cleaners at my uni are outsourced and pretty much all of them are foreigners. The cleaners at my school when I was yet at school were also foreign, though I don't know if they were outsourced or not - I don't think they were. 

As for groups of people known to be dangerous...I wouldn't say there are any dangerous 'groups' per se. There are areas with higher crime rates than others though. This wiki article may be a good starting place if crime rate will dictate where you choose to settle [in London]: https://en.wikipedia.org/wiki/Crime_in_London

Edit: If they take you to Room 101 on arrival, please don't tell them about me. Pls.