My girlfriend witnessed a robbery in London's city center ~2 weeks ago, in the second day of our visit. After that, we saw a homeless guy pooping on a wall; some of them screaming at others.
I didn't feel like I was personally in danger, but I'm also a guy who's lived next to American slums.
I almost had my MacBook stolen out of my backpack from me in London (Shoreditch) a few years in broad daylight - by a black person that didn't even speak English during the attempted robbery.
I'm from Africa so I'm born with the instincts that luckily prevented me from losing anything or getting hurt.
I was just visiting the UK for 3 weeks, but that gave me a perspective how bad immigration laws can turn it into something out of control.
Why does a place like Singapore, where 48% of its workforce are immigrants / expats - not have this problem.
To add to your anecdata, had you visited London during the time of Dickens you would have had your possessions thieved by a charming raffish urchin that spoke English in a dialect you also wouldn't have understood.
I didn't feel like I was personally in danger, but I'm also a guy who's lived next to American slums.