I think you have to remember that LA is a major city (and so, in its own way is San Diego, especially compared to villages and etc around England). There are definitely people I would term homeless in Manchester, though you are right that I see fewer people sleeping rough. (though LOTS selling "Big Issue")
Also, because the weather is so much nicer in San Diego/SoCal, you will see a lot more people out and about, homeless or not, instead of seeking shelter. There is also a tradition in SoCal of looking the other way if people are sleeping in parks/etc that doesn't really exist here in the UK. (Friends of mine in the sheriff's dept in San Diego say they basically check that someone is breathing and not disturbing the peace and then move on).
At least in Manchester, I know the police and social services work diligently to make sure there really aren't a lot of people sleeping rough. They also have, I believe, more efficient systems when it comes to mental health and other disabilities here in the UK, which makes it MUCH MORE likely that someone will get the help they need and not end up homeless in the first place. The mere fact of the NHS may make an impact that you are not aware of.
Also, if you think LA is bad, try San Francisco. They really struggle with an itinerant population there. Also, as far as earthquakes, yeah we were aware of them, but except for the years with a biggish one, everyone pretty much just went about their business as if they weren't going to happen. That's VERY different than having a hurricane season or the harsh winters of the MidWest.