I am always skeptical of comparing numbers of homeless people, as different countries have vastly different methods of measurement.
For instance, New Zealand has the highest number of homeless people per capita. But this is only because in New Zealand, you're basically counted as homeless if you don't have a rental contract or own a house. I was technically homeless for 18 months or so, despite having a roof over my head and being perfectly happy with my living situation, I certainly didn't consider myself homeless until I read an article telling me I was. I still didn't consider myself homeless after reading the article.
Unless we know how the numbers for the UK and California are calculated, there's no point in trying to compare the numbers. Where did you source your numbers from?
I did a bit of research, and while on the face value, the numbers are correct, they aren't really correct at all. There are lies, damn lies, and statistics.
The official number of homeless in the UK is 307,000 [1]. This includes "sleeping rough, or accommodated in temporary housing, bed and breakfast rooms, or hostels". There are only about 4,800 rough sleepers in the UK [2].
In California, there are 113,000 homeless people (2017), of which 88,000 are unsheltered, i.e. sleeping rough. [3, page 25]. HUD defines homeless differently to the UK Govt too, living in a motel or a hotel (if not paid for by the government) is not considered homeless [4]. So there's a whole category of people counted as homeless in the UK, but not the USA.
There are massively more rough sleepers in California than in the UK, even counting for the fact that the UK is much colder than CA.
Anyway, my rather long winded point is that it's difficult to compare numbers between countries, because countries have different ways of counting things.
[1] https://www.theguardian.com/society/2017/nov/08/one-in-every...
[2] https://assets.publishing.service.gov.uk/government/uploads/...
[3] https://www.hudexchange.info/resources/documents/2017-AHAR-P...
[4] https://www.nhchc.org/faq/official-definition-homelessness/