Here is some data that is two years old on CA (and all states in US’ debt to GDP). CA is at 15% of GDP right in line with other states. But, in comparing that to the 81% UK figure you cite above, you would need to factor in CA’s appropriate share of overall USA public debt. Who knows what statistic you’d end up with after doing that? None of this to deny that CA has some serious fiscal problems. But those problems may be more in line with the level of problems other states are facing, which is way different from when you listen to the rantings of the Laura Ingrahams, Shaun Hannitys and Tucker Carlsons and their ilk on Fox News, who try to make it sound like Californians are out in left field living in a different world.
https://www.usgovernmentdebt.us/state_debt_rank
As US states cant avail themselves of bankruptcy their General Obligation constitutional debt interest and principal payments are capped at 15% of tax revenue.
However they can borrow 'unsecured' appropriation debt and they have unfunded liabilities that can grow. If you look below California's unfunded liabilities are about 3x the debt:
http://www.pewtrusts.org/en/multimedia/data-visualizations/2014/fiscal-50#ind4Put simply if you live in California and earn $200k in personal income, your share of the state liabilities in 2013 was ~$60k and presumably its grown in the last 5 years. $200k pretax is not exactly a lot in LA or SF and from that you have to deduct taxes, cost of living, mortgage, retirement savings, child costs, then add on state and federal debt and deficits....
And they have one of the worst fiscal gaps in the US despite being a 'big winner' in this economic cycle...
http://www.pewtrusts.org/en/multimedia/data-visualizations/2014/fiscal-50#ind9Revenue is consistently below spending.
An explosion of underfunding of liabilities is what will probably drive the state finances crisis in a few years time, but as I said, states cant default on constitutional bonds but can default or renegotiate other liabilities including pensions and appropriation debt. Cities can default on GO bonds but often cant default on appropriation bonds like sales tax bonds. Puerto Rico as neither a state nor city has defaulted on everything and has cut taxes for US immigration to almost zero as a way to attract entrepreneurs.