2. US citizen but UK tax resident. So two income tax returns. It's a pain, but thankfully the tax treaty means one offsets the other and it works out in the end.
3. Personally want to live in London
I doubt Brexit's effect on QOL can yet be separated from Covid.
London is a beautiful city with high QoL if you live in a nice part. Much better transit and walk-ability than anywhere in the US except Manhattan. Same language and similar culture for kids in school. Very high quality schools if you live in a good area. Closer timezone to the US than Asia. The government systems are much easier to navigate for an English-speaking American than somewhere like France. Universal healthcare and much lower obvious social issues (i.e. homelessness) than California. Easy and cheap to travel around Europe (Paris is a day trip). Lots of people from all over the world in London. Beautiful cycling in the countryside. Lots of arts and culture in my own language. Great comedy scene and music scene. Endless history to explore.
Other people prefer different places. Lots of places are great. London is a great fit for us.
FACTA has zero impact on my life. Maybe it annoys my bank, but I'm sure they have long figured out how to deal with it. I spend 5 minutes a year sending my accountant a list of checking accounts to report as part of my taxes.
Brexit hasn't really affected me in any way personally, but it might be a bigger factor for others.
London has one of the highest percentages of green space of any large city (33%), excellent public transport, some of the best music and theatre in the world, options in all British sports supplemented by regular visits from American leagues, excellent (though not leading) restaurants and speciality food markets, and more. You can't discount the English language either.
Regarding Brexit, at the moment it's kind of hard to say because COVID (and the ongoing collapse of the government) has muddled its economic impact so much.
The variety is leading though. You’ll have a hard time finding such a variety of high-end restaurants anywhere else.
Sure, if it bothers you to spend ~8k USD/mo on rent, London might not be for you.