I'm from that land called Vancouver, BC. BC in general has NIMBY policies. The cost of construction is also higher. Maybe not as bad as the bay area, but enough to exasperate the problem. BC is also not experiencing a population boom fueled by an economic boom like SF is. It hasn't for the past 20 years, the SFBA has experienced it twice.
London and Vancouver are created by loose investor immigrant laws and tax incentives for the rich. You could get Canadian citizenship as an 'investor' by basically buying a house until very recently. On top of that, you get special tax treatment for the first several years of citizenship if your rich by Canada. Canada's real estate bubble never popped.
China has a large amount of ongoing capital flight, Vancouver is nice and stable, and a relatively quick flight back to China. Vancouver has a huge Chinese immigrant community. All of this combined plus the general Canadian real estate bubble & laws creates Vancouver.
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London is even worse. In the UK they have a nom-dom resident tax law, where you basically just pay a flat tax of ~$40k/yr and have no taxation on your foreign income as a rich person. London is a hub city for the world too. The UK also has very low property tax rates. All of these facts combined create London.
If the UK wants to reverse London, then getting rid of non-dom tax laws, adding wealthy-only property taxes and treating them as normal tax residents will reverse the trend fairly quickly.
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The USA is not an attractive place to be rich resident, unless your wealth comes from a US corporation. The USA has a high relative tax load, citizenship based taxation (only place in the world!), harsh anti-corruption / anti-money laundering laws, a very powerful, wealthy and motivated tax collection agency, a global financial surveillance apparatus, estate tax, a long and annoying immigration process and so on. All of this puts a damper on the super wealthy making another London.
Also the state of a place doesn't really matter, the 'investors' would be interested in molding crack houses. They have the money to turn it into whatever they want. You can do a total rebuild in SF as long as you keep the same square footage. One ancedote is a heavily molded crack shack in my neighborhood listed at ~$600k and sold for ~$900k. Like I was getting an allergy attack just walking into this place for a few minutes. Also see the same thing in Vancouver with this comedy game: http://www.crackshackormansion.com/
In summary: Build more, build tall, we need more 30 story residential apartment complexes! The laws of supply and demand do work! Ideally right beside BART and Muni tunnel stations. The mission became overpriced despite being a bad part of town because of BART.