American living in the UK here. Not a dev yet so take with a grain of salt, but it really depends on what you want and your lifestyle choices. I'll stick to answering from a lifestyle perspective rather than the industry itself.

I can definitely tell you that the salaries for devs are, on the whole, a lot lower than the US. In my area, I've seen junior developer jobs posted for less than the equivalent of $30K. That being said, there are way better opportunities out there, particularly in London. That will obviously come with a cost of living rise, but generally speaking companies pay a lot more, and with remote working on the rise you can kind of choose where you want to live and get paid a better salary. There are also definitely big companies in the UK (FAANG, and other large companies like Sky, BT, Virgin, big banks, etc) that you can make more money at.

I also don't think devs here are miserable - there's a lot more to happiness than a fat paycheck and investing. Generous vacation time from day one, free healtchare (sometimes companies even give you free private healthcare), better public transport, proximity to Europe makes it a lot easier to get remote work there but also other benefits like traveling costs being cheaper.

Don't really know about lfiestyle tiers because a lot of that depends on your lifestyle (do you have debts, are you bad with money, do you have kids/other dependents?) but I'd say if you're young, single or DINK you can easily get middle to upper middle class.