I'm an EU citizen and have lived for 2 years near London (not in London proper, I could never afford that), now relocated to Spain. I have also lived in the US for a bit so hopefully I can dd my two cents with a bit of proper perspective.
London is a city that is booming in an economy that - although damaged by Brexit - is still in pretty good shape. It's a world city, with lots of expats so you should not have trouble finding your community pretty quickly. It also prides itself on a startup scene, although, as Mihai mentioned above, it is nowhere the size and coolness of the Silicon Valley one.
What speaks for London is:
- fantastic public transport, especially when compared to West Coast US cities. You don't really need a car if you live in London, I would even go as far as to say you would be a fool to get one as you'd only be stuck in traffic instead of bypassing it in the subway, overground, trains etc.
- there's always something going on; if it's not an exhibition, festival or concert, it definitely is a market or something similar. There's great parks in and around the city, if you feel like connecting with nature. And there's an endless selection of restaurants, bars, pubs, coffee shops so you will never be bored by your food unless you really want to be
- welfare state with free health care, a proper social security system, a lot of protection in the workplace with mandatory paid vacation etc...
- connections to everywhere, meaning you can travel Europe and a big chunk of the world. London has 6 airports, with at least three of them mostly serving low cost airlines so you can get to most places in Europe for cheap. A train ride on the Eurostar gets you to the center of Paris or Brussels in about 3 hours. And the British rail network is very good too, so you can explore the country comfortably (back to my point about not needing a car). Also, London has a great network of bike lanes and a lot of people actually use them to commute
The cons of London would be:
- the weather. It's not as rainy as I have expected it to be (most of the time you get your rain in the good old English drizzle format) but it does tend to be gray and gloomy for a good chunk of the year. Proper sunshine is rare outside of the summer, to the point where people open the roofs on their convertibles everytime the sun is shining, irrespective of the outside temperature.
- expensive housing. Appartments in London are expensive and small and you need to get ready for that. Unless you are really rich and can afford some of the great appartments in expensive neighborhoods or around the river, you will need to make a choice between expensive and small in London or bigger and cheaper in the suburbs. I have actually lived in a small town in Surrey, where the price of a 1bed closet-sized flat got me a pretty little 3bed house with a small garden. It will all depend on whether you are willing to commute and the coolness factor you want to project (anything outside the M25 ring road is uncool to the cool crowd )
- expensive entertainment. If you do want to go out, you need to pay quite a lot of money. That said, I don't think that would be different to the Bay Area at all.
- Brexit. Covered before by many who understand it better.
Overall, my suggestion would be to do the move. London is great and if you don't like it, you can always move again - with an EU passport to basically any other European city, town or village. Have fun deciding! (